Acqua of the Back Arc

The Acqua of the Back Arc is a major story arc of Toaru Majutsu no Index. Part of the overarching God's Right Seat Arc storyline, it deals primarily with the attempts of the Amakusa Christian Church's, and later, Kanzaki Kaori, at defeating Acqua of the Back before he completes his attempt in destroying, in what he believes to be, the center of the chaos spreading throughout the world, Kamijou Touma and his Imagine Breaker. It features a second infiltration of Academy City by a God's Right Seat member, the first arc to feature a Saint vs. Saint battle, the first physical appearance of God’s Right Seat leader, Fiamma of the Right, and the fate of Vento of the Front after her defeat in the invasion of Academy City.

A subplot in this story arc is Mikoto's realization of Touma's memory loss and her confrontation with him.

Challenge of Acqua of the Back
Acqua of the Back decides to make a move first on his own to end the chaos that his happening in the world, specifically Kamijou Touma and his right hand. After killing Terra of the Left, Acqua of the Back wraps his corpse in a luxurious box. Acqua then sends letters of challenge to both Academy City and the Anglican Church, telling them that he will crush Touma himself and dares them to use all of their power if they intend to stop him. The recipients receive two days prior to the start of the arc. The box is sent along with the letter to the Anglican Church to convince them that the letters are legitimate, and Itsuwa is then called to confirm Terra of the Left's identity, having fought him in Avignon.

Due to Acqua of the Back's method in announcing what he intends to do in a straightforward manner to both the Anglican Church and Academy City, the two forces considered it as a trap. Nevertheless, as Touma was being targeted, the Anglican Church decided that defeating Acqua of the Back in Academy City would be the best. Now that the primary sides of the growing tensions of the world has been drawn, the Russian Orthodox Church allied with the Roman Catholic Church, and the Anglican Church allied with Academy City, the great divide between magic and science sides that normally forbids magicians from acting as a group within Academy City had been broken. Moreover, so great is the threat Acqua of the Back of God's Right Seat, a magician apparently strong enough to execute one of their own, that they would forgo following the rules regarding the separation of the sides and the global chaos that it would cause.

For this cause, the Anglican Church sent the Amakusa Christians to Academy City, likely because they could be used as a scapegoat if things grew inconvenient. Another reason would be is because they originally operated within Japan and knew the land. Itsuwa was later set-up as Touma's bodyguard by her fellow Amakusa Christians in order to get to become closer to Touma.

Misaka Mikoto on Kamijou Touma's memory loss
After discovering Touma's memory loss, Mikoto was in daze as her thoughts are preoccupied about it. She wonders when it occurred and other minute details. She even considers asking for the Queen of Tokiwadai's help, though balks at the thought of being indebted to her and became concerned that she may tamper with his mind.

Itsuwa the bodyguard
Touma and his class were late for lunch because of a comment Touma made in class. The class makes up a complex plan to bypass the school's security and throw off suspicion from them in order to buy food from a nearby store, picking four people to go do it for them: Kamijou Touma, Tsuchimikado Motoharu, Aogami Pierce, and Fukiyose Seiri. Things go awry as Saigo, a large gorilla-like man and part of the school's staff, appears before them, which forces them to split up. Touma is later chased by Saigo.

Itsuwa is first seen outside of Touma's school and discovers much to her surprise, a large man chasing Touma. Mistaking him to be Acqua of the Back as she has never seen him before, she tries to attack him. The chase is brought to an end, and Saigo is apparently forced into a retreat due to "health" reasons, much to Touma's relief, allowing him get lunch. After class, Touma speaks to a saddened Itsuwa, embarrassed by her mistake. Itsuwa asks Touma if he knows who Acqua of the Back is, to which he informs her that he has met him back in September 30. Itsuwa later tells Touma that Acqua's target is him, saying that the Anglican Church and Academy City has received letters of challenge. Touma is doubtful with Acqua of the Back going after him after Vento of the Front telling him on how the Pope required many documents to have people attack Academy City. Itsuwa points out that it is because that Touma had a hand in stopping their nefarious plots that they have Acqua of the Back after him. Itsuwa notes that the British Library is checking on his history, and though there is not much to go on, concludes that he must be powerful to be included in God's Right Seat and has the strength of a Saint, and wishes for Kanzaki to aid them. Itsuwa points out that fighting in Academy City should be a great advantage, as both Vento of the Front and Terra of the Left gave them opportunity to fight back due to interference of science. Still though, Touma says that he doubts that it would be that simple. Itsuwa tries to reply but can only mince words and in a flustered states says that she will be protecting him as they have been given orders by the Anglican Church. Touma asks again why Itsuwa is there with him, as he does not believe what he has heard. She then reiterates that she is his bodyguard.

Meanwhile, the Amakusa Christians are observing Itsuwa's progress with Touma, but later gets bored of her slow pace. They later plan on having Touma's head plant on Itsuwa's breasts. At the same time as this, Misaka Mikoto's thoughts are still preoccupied with Touma's memory loss and notice them kicking the ball. Saiji is successful and Touma's head falls upon Itsuwa's breast. Seeing this, an irate Mikoto shouts at them and begins emitting powerful electricity. But before Mikoto can do anything about them, Saiji and the other later leave quickly without a trace. Meanwhile, Itsuwa doesn't know what to do, she is flustered and tries to rub the part of Touma's head. Mikoto dashes towards Touma and attacks him using her Electromaster powers. In order to fulfil her job, Itsuwa started to assemble her spear, and in order to stop her, Kamijou hugged her, leading to Mikoto becoming angry again, forcing Touma run around Academy City.

Itsuwa arrives in Touma's dormitory, leading to Index questioning why she is there. Itsuwa tries to explain to Index regarding God's Right Seat but is told to be quiet about it by Touma around Index as he doesn't want her involved. Itsuwa understands but is flustered after Touma took her away by the wrist. As Index becomes incensed, Itsuwa notices Sphynx, and gives it high-class cat food as a present. Touma notices the various foodstuffs in Itsuwa's bag, to which she claims is that despite her being a bodyguard, she shouldn't just eat and drink for free, telling Touma to entrust her with the housework. The atmosphere completely changes as Touma becomes enthusiastic, much to Index's surprise. He shows Itsuwa the wok for her to use Itsuwa's presence greatly lightens Touma's mood. Itsuwa then begins cooking much to Index's excitement, and tries to bother Itsuwa for it. Having none of it, Touma shouts at Index to "stop ruining a man's dream," and tells Index to follow Itsuwa's example. He forces Index to clean the bathroom while they wait for Itsuwa to finish her cooking. As Touma begins to do some housework himself, there Tsuchimikado Maika smells the scrumptious smell of the miso soup. Maika is able to automatically pinpoint the special ingredients Itsuwa made, much to her bewilderment, and acknowledges Itsuwa's skill. Seeing to one up her, she returns to Motoharu's room and throws away the cream stew she prepared for his dinner, in order for her to make a much better miso soup. Itsuwa hears Motoharu's cries, and recognizes it as from the incident in Avignon but is unable to confirm it. After dinner, Itsuwa suggests going to School District 22 for a bath after Index destroys the bathtub in her attempt to clean it. With the last bus already leaving, Itsuwa takes Touma and Index using a rented motorcycle with a sidecar, leaving Sphynx with an expensive scratchboard to play with. With Itsuwa speeding, Touma is forced to grab Itsuwa's waist even tighter as he is riding behind her, making her happy. They later enter the district to which Itsuwa relates to Touma on her surprising knowledge with the City. They then enter the Spa Resort Serene Springs, separated by sex, as such only Index and Itsuwa bathe together. Misaka Mikoto is also there, trying to collect 10 cards with stamps on them in order to avail an out-of-bath Gekota, and the only way to do that is to bathe as much as she can. She comes upon them and later bathes with them. Mikoto becomes noisy in the bath but is promptly scolded by Index. Mikoto recognizes Itsuwa as the "girl who got hugged by that idiot because of some strange soccer ball," making Itsuwa embarrassed and ebb into the water. Mikoto starts to wonder if the two know about his memory loss and while pondering about the issue again, she faints in the bath and Itsuwa and Index is forced to call the staff to have her resuscitated.

Amakusa Christian Church's first battle against Acqua of the Back
Leaving Index back in the bathhouse who wanted to taste some of its food there, Itsuwa accompanies Touma for a walk. Here, she and Touma talk about the Amakusa and their life in England, and later moves to Itsuwa and Kanzaki's clothing. Itsuwa then talks using the metaphor of Vega and Altair (Orihime and Hikoboshi respectively, the star-crossed lovers of the Tanabata) to describe her relationship with Touma, not being able to see him, though Touma doesn't understand.

Meanwhile, the Amakusa Christians are also in School District 22, scattered to have Touma be the center, and continuing their protection of Touma from far away. Ushibuka later comes up to him and asks if Acqua of the Back really is going to attack, to which Saiji offers his observation to him. Here, Saiji confirms that Touma is worth protecting not because his is their savior, but because he is also their comrade as well. Then Saiji wonders about the value of Kamijou Touma to Academy City, to the Anglican Church, and to the God's Right Seat, to which Saiji concludes that there are a lot of hidden information regarding Touma. However, Saiji suddenly feels constricted, and discovers that he and their comrades are the only ones who are walking in the street. He prepares his comrades for battle but they are too late, Acqua of the Back utterly defeats them, though doesn't kill them as they're not the ones he intends to kill.

Back with Itsuwa and Touma, the both of them are unaware that Acqua of the Back has appeared before the Amakusa Christians and utterly defeated them. The two continue to talk, with the topic flowing towards Acqua of the Back, God's Right Seat, and the role of the Amakusa Christians in protecting them. They later move to a bridge, and after Itsuwa blushes at the thought of them being alone together, they note their queer situation. There is literally nobody other than the two of them. And there came steps from the darkness and it showed Acqua of the Back. He reminds them of his warning, but since they've chosen this path he tells them that he will use all of his strength to crush them. Acqua of the Back says to him that he has come to crush him, the reason for all the unrest in the world. Touma calls him out on this, and references what they did to Avignon. But Acqua would not yield, and points that what happened there only happened because of Touma and his Imagine Breaker. He then tells Touma to hand over his right hand if he wants him to spare his life. Itsuwa asks about the Amakusa Christian, to which Acqua says that he did not kill them for what he wants is not them. Despite Touma and Itsuwa's concentration, they are caught unawares. Acqua sends Itsuwa flying with a single hit, allowing him to attack Touma with his giant mace that he summoned. Fortunately, Itsuwa somehow flings her bag between them, sparing Touma for the moment from the large mace despite the debris hitting him. But the sheer power of the blow flings Touma high up and then hitting the metal frame of the bridge, making him fall. Itsuwa hastily assembles her Friulian Spear, and she prepares to fight despite being damaged from the previous attack. Acqua of the Back points out the folly of challenging the man whom her entire group couldn't beat, but Itsuwa would not yield. Seeing this, Touma tries to stand but is unable to as Itsuwa and Acqua fight in high speed. But it is quickly over as it had begun, and Itsuwa is hit with the mace and then tossed towards Touma, hitting him. Touma understands the situation, which he could not hope to match against Acqua even after he fought Vento of the Front and Terra of the Left. Acqua of the Back once again asks for his right hand, but Touma doesn't cooperate, prompting Acqua to attack him again.

Itsuwa regains consciousness after Acqua of the Back pummels Touma to the point of drawing blood. Itsuwa appears before a bloody mess. She is horrified by this, at which point Acqua notices her, telling her to move or she'll be caught by the tremendous power of his attack on Touma. Itsuwa breaks down and desperately tries to heal Touma in spite of his right hand. But it is vain due to the Imagine Breaker, and Itsuwa can only cry. Acqua of the Back tells her that it is enough, and steps on her back, silencing her cries and moans. As Acqua tries to come towards Touma, a screaming Itsuwa raises up her arms against Acqua again, trying to protect Touma. Acqua sighs as he and Itsuwa prepare for the inevitable conclusion of their clash. However, before Acqua can strike him down, Touma once again stands by her side. He thanks Itsuwa for the healing magic despite her knowing it did not work, and in her shock, cannot stop him from going towards Acqua of the Back, to protect her. To save Itsuwa, Touma alone stands against Acqua, impressing him. Acqua of the Back deals a horrendous blow upon Kamijou Touma, but does not kill. Touma is hung upon the mace. Acqua of the Back smiles at Touma's courage, and tells Itsuwa that he is giving them one day more. He tells her that he will give them one more day to choose whether or not to give Touma's right hand, and if they did, he will spare their life. Afterwards, Acqua swings his mace that had Touma on it, flinging from the bridge and into the cold water at high speed, and finally sinking after bouncing from it several times. Without confirming whether his target is dead, Acqua turns his back on Itsuwa for one day more.

Amakusa Christians recover and regroup
Touma survives but with horrific injuries. He is later taken into School District 22 hospital, and where the Amakusa gather once again. In the hospital, Touma is able to rest thanks to the anesthetic. Index later sleeps next to him, and is allowed by the doctor as he deems it to be better if she is alone with him. After hearing the doctor's diagnosis in a hospital in School District 22, Saiji says that they should at least apologize to him, but the doctor vetoes the thought. Powerless, the Amakusa can only regret that they have failed to protect Touma. Saiji sees Itsuwa sitting in the corner sulking and dwelling on her own failure of protecting Touma, to which an irritated Saiji raised her and told her on how Touma could've protected a woman such as her. Before Itsuwa could raise her hand on him in anger, Saiji throws her down, and tells that Acqua of the Back will come back and asks if they should continue sulking or stand up and confront Acqua of the Back and defend Touma. He tells Itsuwa that if she wants to apologize to Touma, she must make it so that she is someone who was worth protected by him. He asks his comrades if there is anyone who feels the same way as Itsuwa before as so he could wake them up. Predictably, no one responds, and they once again prepares to take on Acqua of the Back.

Aid from Necessarius
While all this is going on, Orsola Aquinas who is also working in the British Library in order to figure out Acqua of the Back, and in turn, discover his weaknesses, contacts Sherry Cromwell as she is giving a lecture in the Royal Academy of Arts, reporting on her findings. Orsola mentions that Acqua has been witnessed as someone worked previously in England, and is referred to as a Knight, which worries Sherry as it would mean Academy City might have to question the Anglican Church's role with regards to a Knight of England. Orsola notes however, that there is no record of him in the Buckingham Palace, to which Sherry guessed that the information might be false. Orsola however says that it is true that he is not in the list of knights, but she tells Sherry that for someone to be a knight there should be some shield heraldry that symbolizes their family background. Orsola then says that there has been a cancelled order from a heraldry manufacturer outside of London from an unknown person. Sherry is intrigued, as that means that she could extrapolate information regarding the mysterious Acqua of the Back using the heraldry. Orsola sends a fax copy of the heraldry pattern to Sherry. With a simple look, Sherry recognizes the meaning of the symbolic animals and legendary creatures in the heraldry, and says that the person was unhappy at being chosen as a knight.

She states on how the Royal Family has invited him, tried to reject the offer but failed, and simply accepted the title begrudgingly. She tells Orsola that the man must've been an active fighter as a free man before becoming a Knight. He was a mercenary that was later appointed as a Knight. Sherry then discovers that the order was cancelled ten years ago. Sherry says to Orsola that the person was born in England, was a mercenary that greatly contributed to Great Britain, and was primed to be nominated into being a Knight, and orders her to search for information on English mercenaries. Orsola continues her investigation and later discovers Acqua of the Back's identity.

Final preparations
They later have the 50-strong Amakusa Christians in an alleyway in School District 22, awaiting the intelligence from the Anglican Church regarding Acqua of the Back. Saiji reports to his comrades that Acqua of the Back is currently fighting unmanned robots, and that they should prepare first before challenging him. As the Amakusa prepares for their battle against Acqua of the Back, Saiji turns to find Itsuwa, finding out that she is intensively strengthening her weapon beyond normal, asking for them to wait three more hours before making their move.. This makes both Saiji and Ushibuka both unnerved, with the former thinking that he went overboard with her. Orsola later calls Tatemiya Saiji, who begins preparations to assault Acqua of the Back anew after his attack on Touma and Itsuwa. Saiji is relieved as Orsola's call manages to lighten up the tension that Itsuwa's preparation has brought. Here, Orsola tells them the latest information about Acqua of the Back. She tells them that Acqua of the Back's real name is William Orwell, his past religious affiliation, and his magic name, Flere210. She also tells of his accomplishments as a mercenary, which is countless as the time is short, as well as the kind things he does in the battlefields when not fighting. However, she tells them that no information about him can be called a weakness. Orsola continues, saying that Acqua disappeared a week before he was to become a knight, leaving the heraldry prepared for him, as well as the fact that he is self-taught, making his own fighting style, and the weapon that he uses, a 5 meter long iron rod that looks like a lance. Finally, Orsola tells Saiji that Acqua does not run in battle, but seems to glide on the floor, using a water spell. After their conversation and still learning no weakness and discovered that Acqua of the Back is more powerful than they thought, Saiji and the others are still determined to protect Touma from him, and the Amakusa once again prepares for the storm that they shall meet head on.

Academy City response to Acqua of the Back
Meanwhile on the third level of School District 22, Acqua of the Back is in a nature park, speaking to the Pope using magic through a phone, likely to disguise it. He warns the Pope that despite being in the park away from all the technology, there is still a chance that they're being spied on like the Amakusa Christians. The Pope asks about Acqua of the Back not killing Touma but just taking his right hand, saying that he recalls Vento of the Front telling him that God's Right Seat won't change their goals. Acqua of the Back however tells him that is a problem in Vento of the Front's character as God's Right Seat change according to how the situations plan out, and Terra of the Left himself went overboard. Pope tells Acqua of the Back that what he is doing now is the better method. However, he says to him that if Touma really intends on defying God, they need to kill him, though says that since Touma isn't all familiar with God he is against killing it because of this reason, which Vento of the Front thought otherwise. Acqua of the Back tells the Pope that he is neither a good man nor a philanthropist, saying that if it comes to the point where he needs to kill Touma he will still do it, though there is still a possibility that he doesn't need to kill at all. The Pope then relates on how though God's Right Seat was set-up as a negotiating tool to enter the enemy's stronghold, he can only watch from the Vatican. After pondering the purpose of God's Right Seat in the Roman Catholic Church, Acqua of the Back tells the Pope that the next time he contacts him it will all be over, and that whether the target will still be alive after that then he'll tell him later.

Just then, he hears a loud noise, tucks away the phone as he senses danger, and jumps away from where he is standing. The effects are instantaneous as the place he was previously in is ripped to shreds, an attack from Mimosa. Although not knowing what attacked him, he successfully hypothesizes that strange particles have been scattered into the air that then decomposes the contents it touches. Acqua of the Back then also notices that School District 22 has announced that the 3rd level district is low on air and advises people to evacuate designated buildings and wear their oxygen flasks, which Acqua of the Back notes later that Academy City is testing his battle strength and at the same time create a battlefield without bloodshed.

Furthermore, he then notice unmanned Powered Suits approaching him. The Pope tells him to try and not to kill anyone, though he tells him that they seem to be unmanned. Acqua of the Back is amused by this, their mobilization and despite the technology Acqua of the Back easily overpowers them, and merely seeing the excursion as a way to pass the time until the Amakusa-Style Remix of Church have come to their decision.

His endeavors have allowed him to move to quite a bit of distance from the park where the Mimosa first attacked him. Just before the Amakusa confront him, Acqua of the Back has taken down a total of 8 self-propelled Mimosas, 17 armored vehicles, 38 androids, all unmanned. His battle is also noticed by the Amakusa Christians as they regroup.

Amakusa Christian Church's second battle with Acqua of the Back
By 3 AM in the morning, Acqua of the Back's little excursion has led him to a metal bridge in School District 22, a deliberate choice of his as he attempts to find a human controller to no avail. Academy City forces later retreat after an hour of fighting. Seeing this, Acqua of the Back admits that his enemies are not as tactless and dumb as he expected. Acaqua of the Back tells himself that it would have been easy if he had Vento of the Front's Divine Punishment. Acqua of the Back then checks on the time and sees that there is still 19 hours left before the Imagine Breaker deal, but there was not time to wait as he sees that the Amakusa Christians have already made their decision as they approach him.

He asks them if they are prepared for it, to which Saiji says that it is because he gave them a tough question they couldn't decide on and as such quickly settled it. Hearing this, Acqua of the Back then summons his large mace before them. He warns them that they are against the 2 billion followers of the Roman Catholic Church, and losing a single arm just so they can escape should be a bargain. Saiji says that the Church isn't the enemy but people like him who treat ordinary people as bait and manipulate them when they continue to believe in God, though Acqua of the Back just scoffs at this reply. He says to them that he isn't bothered by their decision and on the contrary, it is them that should be worrying as they gave up on their only chance at surviving. He reiterates by pointing out that he is one of the Saints and is a member of God's RIght Seat, though he says he is willing to face all of them and look forward to it if they continue to defy him and protect Touma even after hearing all of those. He then says that at the end that he will win. As he prepares to attack them, he says that is hoping that they would at least force him to use his trump card, but before he could finish he is attacked by Itsuwa with magic, creating a powerful explosion. This surprises Saiji, though Itsuwa does not respond to him as she faces an unhurt Acqua of the Back telling her that she should have listen to people's words until the end. However, Itsuwa is having none of it, and says that if he has something to say then he should say it later after he beat him to a pulp and if his jaw isn't broken. The males are in awe and shock at her action while the females are silent, intrigued by her actions.

Itsuwa ignores the commotion, continuing instead to glare at Acqua of the Back. He is impressed by her bravery but asks her if she can back her words with action. Itsuwa says that even if they are reduced to "slice of meat" they'll let him regret it all he did after beating to a meat pulp. And with the cries of her comrades regarding her outrageous claims, Itsuwa and Acqua of the Back clash in battle. Itsuwa was the one who contended against Acqua of the Back directly, while the rest of the Amakusa Christians support her by continuously changing the focal point, it was a spell that increases visual activity and mobility, allowing for Itsuwa to match up to Acqua of the Back's speed.

After he begins catching up to Itsuwa, the Amakusa Christians the planted wires around him, though his strength allowed him to resist a little, they eventually activated the spell, and Acqua of the Back's body is engulfed in a red mist prison that explodes from within. And yet Acqua of the Back continues persisting, referring to his Divine Mother's Worship which allows punishment upon him to be weakened, basically making their attacks meaningless. Acqua of the Back then tries to attack, forcing the Amakusa Christians to retreat.

They later regroup 300 meters away from the metal bridge where they fought Acqua of the Back. With Acqua of the Back always breaking their formation, Saiji concludes that they should better use the weakness of the Saints, since they have properties similar to the Son of God, then they will be weak to things that have similar properties to those that have executed him. Feeling Acqua of the Back coming, Saiji tells his comrades to get into position, they create an entrance to the space between the third and fourth level, a hydroelectric turbine power plant. Saiji and the others set-up wires and trap spells down below in order to delay him long enough for them to set-up their counter against him. With Acqua of the Back's arrival, Itsuwa becomes the focal point of their formation once again. Here, Itsuwa discovers that despite being a member of God's Right Seat, he is still able to use normal spells due to his Divine Mother's Worship. Thus, being aligned to water, Itsuwa runs for her life as everything around her is a weapon for Acqua of the Back. Itsuwa is later blown down the screen that acts as the 4th level's sky. Both draw their weapons, and Acqua of the Back prepares to end the battle, however, the other Amakusa Christians finally arrive through the entrances. Battered and injured, the Amakusa Christians face down Acqua of the Back.

While Itsuwa fights Acqua of the Back directly, other members attack him in every direction, however, they are still no match for his impeccable power, blowing them away. Acqua continues his attack and swings down upon Itsuwa, who barely escapes by using her sweater as a scapegoat. Itsuwa trades blows with Acqua of the Back, but the both of them know that she is getting slower. The other members support her by attacking once again in all angles, but they are once again blown away. While Acqua of the Back is busy, Saiji calls out to Itsuwa if the spell is ready yet, but their conversation is quickly interrupted after as Acqua of the Back attacks him. Saiji manages to block the strike but is still blown away by him. Acqua of the Back presses on with the offensive, slowly sapping Itsuwa's strength and endurance blow by blow they trade. However, as he moves, he soon discovers that the spell that allows him to glide and move in extreme speeds have been mixed in with some substance by the Amakusa Christians, moreover, the shockwaves brought about by this disruption rips apart the screen that he is standing on.

With this gap, Itsuwa smiles and charges forward with her spear, seeing this, Acqua of the Back dodges for the first time in their battle. He jumps up, but that is enough for Itsuwa, who calls out to her Saiji and the rest of their comrades. They respond and redo their formation again with Itsuwa being the focal point. With the tube spear to reduce the friction between the spear and palm, Ituswa strikes Acqua of the Back with her spear. It turns into lightning in the shape of a cross, the execution method that was used on Son of God, therefore the weakness of the Saints that has their bodies having the same properties as that of the Son of God, strikes down on Acqua of the Back. It was the Saint Destroyer, the culmination of the Amakusa Christians efforts to truly know their Priestess, Kanzaki Kaori, a Saint herself, in order for them truly be by their side.

But not a scratch came upon Acqua of the Back, for he had grabbed Itsuwa's spear before it hit and modified and weakening it. Slightly impressed, Acqua of the Back tells them that if he was a mere Saint then it would most likely finish him off, if not for the fact that he is also a member of God's Right Seat. He then blows her down to the 4th level, ripping the screen up. As she falls, Itsuwa and the others use barrier spells for her descent, but this was not enough, and Itsuwa falls to the floor below. Bored of the battle, Acqua descends down to her and asks Itsuwa if she is willing to hand Touma's right hand over to him or choose being killed. But Itsuwa answers with her actions, standing up once again to challenge him. Seeing this, Acqua of the Back summons the water from the 4th level and forms it into a hammer. He tries to strike it down upon Itsuwa, but ceases before doing so. Acqua of the Back notices something and disperses the spell. He tells her that she should thank her master and leaves, for Kanzaki Kaori has arrived to challenge him.

About 200 meters away from where Itsuwa landed, Kanzaki faces Acqua of the Back. He notes that he has heard that she hates fighting, and asks her if she has any intentions of fighting against him, to which Kanzaki says that she will. Kanzaki explains that it is her personality and it is borne by her magic name, and seeing her comrades being beaten one by one, she cannot allow it carry on. She then says that she will not allow their efforts, Touma and the Amakusa Christians, to go to waste. Thus begins the battle against Saint and Saint.

Saint vs. Saint: Kanzaki Kaori vs. Acqua of the Back
And so the two Saints clashed and their weapons met in flashes and explosions and in great speeds, displaying their awesome power, and the ground shook as they stood. And they exchanged magic magic and the other countered them with their own as if the both of them knew just as much as the other. And for Kanzaki who has knowledge of spells from different religions, made great use of it against Acqua of the Back. Acqua of the Back notes on how much she and the Amakusa will go through for the sake of Touma, impressing him. However, he warns that since she has become his enemy it has been already determined that she will be defeated by his hand. Then Acqua of the Back sends out a large pillar of water from the nearby stream and smashes it down upon Kanzaki, whom easily cuts it down with the power of Nanasen, nothing that she will be insulted if he thinks that is all her full power.

With that she uses Nanasen to assault him in all angles, and though he can easily respond to them, Kanzaki made it so that the wires formed a magic circle, activating a magic spell and engulfed his entire body with flames. After three explosions, Kanzaki assaulted him with a slash of her blade. But he escaped though not wholly for there was a cut on his face, the first time since the battles in School District 22 against him began.

Unperturbed by this, he notes on how she and the Amakusa Christians are similar, though the difference between them is because she had talent. Using the blood from his wound he wrote down something on his mace. Although stunned because of his words for a moment, Kanzaki then tells him that she hopes that she can correct his words, and sheathed back her sword into its scabbard, in preparation of drawing it back again. She tells them that though they can't do things like the Yuisen, they know a few other things as well that have been passed down the ancestors of the Amakusa, concluding that these weren't made by talent but by the result of history. She tells Acqua of the Back that Amakusa is her school, her teacher, and comrades, and that she will never agree with what he just said. Kanzaki tells him this knowing that Touma was in similar situation as her when they first met, as if it was punishment on herself. Unmoved, Acqua of the Back says that to be angry over something that small is immature, and mocks their beliefs being put in the battlefields where there are no rules. Kanzaki tells him that it just his reasoning, though he replies that they are walking into his battlefield, and wonders what master pulled Touma up like Kanzaki had pulled the Amakusa. Hearing enough and with no warning at all, Kanzaki sped towards him and tried to cut him down with her blade. An angry Kanzaki shouts at Acqua of the Back, saying that if he knew all that then why he treated him as someone who got involved, asking him why he used the full power of a Saint to attack him, knowing full well that she too had hurt Touma the same way. However, he remained calm, and basically tells that he is confident in what he is doing is in the right and that he needs no excuses for the path he has chosen. As Saints collide, the blood markings on the mace activated, forcing Kanzaki to increase the distance between them. Acqua of the Back then says to her to show him her belief not via shallow words but by her blade. There, the weight and pressure extered by the mace exponentially, and the two Saints once again clashed.

Using all of the water available in the 4th level of School District 22, Acqua of the Back creates a complex magic circle made of water that allows him to barrage Kanzaki with attack after attack of water shaped by magic of many forms, from ice spears to water whips. Moreover, he himself attacked Kanzaki. However, Kanzaki persists using her wires to cut them down and make barriers, even after Acqua of the Back predicted that she would be too slow to dodge and block. As their clash continue, Kanzaki notices that Acqua of the Back is using Divine Mother's Mercy. As they continue doing battle, Kanzaki grits her teeth though not just because of the pain, but because of Acqua of the Back's blatant misuse of the Divine Mother's Mercy in acts of violence. As they continue to clash, Acqua of the Back states that he is impressed by her managing to endure the great magic circle that spread out to two kilometers in diameter, though asks if her body is at its limit. This surprises Kanzaki, though she once again had her weapon clashed with his, though she is reaching her limit, what's more, her technique, the Yuisen, is not suited for long-term combat, and that it should only be used when a kill was guaranteed. With Acqua of the Back, a one-hit kill is impossible. In contrast to Kanzaki's obvious signs of wear, Acqua of the Back remains smiling. Kanzaki recalls her battle with Misha Kreutzev, and in contrast to her, Kanzaki feels that Acqua of the Back seems to have an even greater power coming from him. Believing this, Kanzaki deems that Acqua of the Back should have destroyed himself with that overwhelming power coming from him. Then as she ponders this, Acqua of the Back once again to attack with his mace. Kanzaki has her sword meet with it, but so great was the power that the ground that she stood on shattered as she endured the attack.

Kanzaki tries to attack with her sword but Acqua of the Back easily blocks it. Having enough of the battle, Acqua of the Back comments on though fighting another Saint is excellent sport as he hasn't done so for three years, he still has work to do and he doesn't have the time. Before Kanzaki could respond, Acqua of the Back jumped up into the air with the moon of 4th level's artificial sky behind him. Kanzaki, being too damaged by their battle is unable to close the distance and is helpless against what Acqua of the Back is about to do. He then recites an incantation of the Divine Mother and the artificial moon behind him begins to glow, and the screen that creates the artificial sky begins breaking apart. After finishing the incantation, Acqua of the Back explosively dives from the screen, destroying the entire artificial sky in the process, and dropped down towards Kanzaki in order to crush her with the mace. So great was the release of power that everything within a 100 meter radius of the 4th level of School District 22 collapse and fell down to the 5th level. However, as the dust settles, Kanzaki has endured Acqua of the Back's hit with her sword, though the ground below her feet did not, and she has fallen down. Moreover, the impact was still strong enough to thoroughly damage her.

There in the 5th level, lying on top of debris, she once again stands up, enraged at Acqua of the Back, though relieved that there were no bystanders around. She lifts herself up again and picks up her fallen sword. Meanwhile, Acqua of the Back who is unscathed, asks her where the Imagine Breakers is, wondering if he destroys every level he may be able to find him. This does not sit well for Kanzaki, and screams his name. Filled with wrath, the injured Kanzaki grips her sword tightly and speeds towards him. There, their techniques once again clash, with Kanzaki swinging her sword on him and moving her wires to create magic circles that can discharge fire or ice spells, while Acqua of the Back merely blocks her sword with his mace and maintained a steady barrage of attacks. He absorbs the moonlight on the screen an attribute of the Power of God, the archangel he is aligned to, to increase his attack power. Moreover, he uses the Divine Mother's Mercy to allow him to surpass the limits of God's Right Seat on not being able to use ordinary magic.

In spite of her attacks, Kanzaki is clearly outmatched by Acqua of the Back, and her injuries bring her no relief, as she had reached her limit, and all her trump cards have been played in the protracted battle between Saints. In this dark hour, however, Kanzaki can only recall Touma's words back when she herself mercilessly beat him and where he continued to defy her, telling what she was hiding from herself, and that is she gained power because there was something she wanted to protect. She recalls how the powerless Touma stood up against someone whom he possibly could not defeat in order to protect Itsuwa, and believes that it would never lose to those who thought they were "God-chosen" and acted above others. And so, Kanzaki continued to battle, as she is fueled with the desire in not letting Acqua of the Back get away from her unscathed.

Meanwhile, the Amakusa Christians watch from the distance as Acqua of the Back continues to dominate Kanzaki and the battle falling down into the 5th level of School District 22. Right now, they are powerless, and the injuries and battering they've incurred from Acqua of the Back playing with them have left some of them to lose their grip on their weapons. Moreover, because Kanzaki is protecting them, it seems that their efforts have come to not, for once again their Priestess stand in front of them to protect and not them standing beside her to aid her. The Amakusa sink into despair as the battle between the Saints continue.

Kamijou Touma and Misaka Mikoto
Touma's eyes flicker and finally awaken, finding himself in the hospital where the Amakusa Christians left him to rest. He finds Index sleeping next to him. Seeing her waiting on him pained him, though Touma does not sit idly by. What little strength he had he uses to move his body for there is much to be done. He believed in the skills of the Amakusa Christians but even he doubts that they can be victorious against Acqua of the Back, and opts to aid them even in his condition. In spite of just being an ordinary high school student, he just could not stand by and let the Amakusa Christians fight alone. However, he knows of the power of his Imagine Breaker that it can destroy even God's miracles, believing that even with that little power he could cause a drastic change in the situation that could be used against Acqua of the Back. After confirming that he still has his right hand, he glanced at Index and apologizes to her, and tells her that he will continue to apologize more afterwards. He later leaves as he is and somehow escapes the hospital.

Misaka Mikoto who was stuck in the bathhouse building because of the "no oxygen alert" was finally able to go out of the building. She comes upon the gate that leads out from School District 22 but there has been a malfunction, making the people unable to get out. While thinking about what trouble Touma is in, she slightly loses control of her power, creating a spark, embarrassing her. Here, it was the apparent effects of a "people clearing field" set up by a magician, influencing her actions. It works and Mikoto gives up to get back to the surface and decides to go to a hotel in the 7th level of the district. After getting there, she comes upon in the darkness a figure that is walking strangely. She narrows her eyes and as the figure appeared below a streetlight, she discovers that it is Touma. Though recognizes it is Mikoto, he continues moving regardless but falls down, forcing Mikoto to catch him. Touma tells her that he has to go for "they" (the Amakusa Christians) may still be fighting. Seeing him in his current form, Mikoto deems that Touma had been in countless battles that she doesn't know, and that the memory loss may truly be legitimate because of the fact that in these battles, Touma has been beaten to an extent that could allow for a memory loss to occur. Touma tries to once again move, but Mikoto holds his arm, preventing him from doing so, and he notices it. Mikoto confronts Touma about his memory loss, shocking him and confirming to Mikoto that it is both real and that he still had memories of his battle with Accelerator and his saving of the Sisters. She asks him why he doesn't ask her or anyone else to help, mirroring the events of that fateful day where Touma trampled on her feelings in order to save her and the Sisters from Accelerator. She volunteers to confront the person that he is going to fight against in his place, and that he should at least experience what she had felt back then, powerless and can only watch from a safe distance. Finally, she asks him why he doesn't apply his own beliefs on himself, and why he alone is the one not asking for help.

Mikoto is relieved that Touma still remembers what he had done for her and her Sisters, but also felt disgusted by her own selfishness, using the situation as a catharsis to wipe away her anxieties instead of worrying about his condition. As Mikoto frantically tries to search for a nearby hospital on her phone, Touma finally responds. Seeing this, as if Touma could break away from her at any moment in spite of his condition, Mikoto continues to grab his arm as he speaks. He acknowledges her finding about his memories but tells her she is wrong. He tells her that he doubts he has put himself in so much danger that he lost his memories because of it. He tells her that in spite of his lost memories it is because of them that he is able to stand before her, telling her that his past self still motivates his current self as he still resides within his heart, and that remembering or not is irrelevant as he still understand what he needs to do. Touma here shows that he holds something that he could not even grasp as a source of provide, though it is because of these beliefs that allows him to move without regrets. It is something that does not compel but drives him, as it is not he must do so but is that he should. He apologizes to Mikoto and tells her to go back, and at that point Mikoto was affixed at his words and had let go of her grasp of him. Finally, he tells her that even if he loses his memories it doesn't change what he needs to do as Kamijou Touma isn't the kind of person that lets something like memory loss stop him.

And thus Kamijou Touma continues on forward, with Mikoto only being able to look at his disappearing figure in spite the fact that she had the power to stop him. She did not know what to do, despite the fact that she knows he didn't say that she was wrong, or that she shouldn't go with him in his battlefield. However, she knows that Touma did not lie, and that there was some meaning to Touma standing and continuing on his own. Realizing this, she sees that letting Touma go and seeing him off is the right answer, that she could only hope that he will come in one piece, and any other option would just trample on his wishes. But still she couldn't accept what Touma was doing, though she is unable to stop him. And as Touma disappeared into the veil of darkness into battlefields not yet known by her, Mikoto realizes that she couldn't stop Touma because of her spirit had been beaten back by his actions, but because she realized her feelings for him. This overwhelming emotion pressed upon Mikoto's heart, and left her unable to do anything.

Amakusa Christian Church's third battle against Acqua of the Back
Kanzaki is at her limit, and is slowly falling back, and her strength wavering. As Kanzaki looks on, Acqua of the Back continues to look down on her and the Amakusa Christians. There is just too much of a difference between her and the pure talent and genius of Acqua of the Back. He points out that he will crush them for they chose to fight him regardless if they are incapable of fighting back, to which Acqua of the Back just looks down upon them. Kanzaki steps inside the shoes of Acqua of the Back and discovers that they are not that different. She is a Saint, and though she cared for her comrades, she looked down upon them, never letting stand beside her in the battlefield, trying to control everyone so that they would not get hurt. With that Kanzaki realizes her own foolishness and her own weaknesse. Here, she discovers that she still had the power to challenge Acqua all along.

Upon the 4th level the Amakusa Christians heard her call from below; Kanzaki calls out to her comrades to lend her their power. And their bodies trembled in their place, and those who have lost their weapons in their frailty pick them up once more, for their leader has come "a calling". They were now recognized by their Priestess, they are no longer a burden to her. Now gathered, Saiji gives his last instructions as the Substitute Supreme Pontiff, their Priestess, to where the Amakusa should go.

They gather around their Priestess with a jubilant shout, surprising Acqua of the Back. He tells her if he is willing to have the weak help her to save her own life. However, Kanzaki reveals that she was the weak one, for not believing in her Church's abilities and strengths. Because of these insecurities in wanting to protect what she deemed weak she made mistakes allowed holes in their formations and putting her comrades in danger. And that is why she called her Church back to her, for she can trust herself to them, to utilize their ability to the maximum. Acqua of the Back however, is unperturbed, and Kanzaki meets his rod with her sword, however, this time the Amakusa Christians cast defensive spells to reduce its impact. As their battle goes on, Kanzaki points out that Acqua of the Back's strengths are also his weakness. The reason why he can indiscriminately use his Saint powers without his body overloading is because of his Divine Mother's Worship. However, this makes his weakness be the Saint Destroyer, as proven by the fact that was the only time he used a magical defense. Kanzaki says that to regular Saints, it will only cause them to be unable to move for several seconds, but for someone like Acqua of the Back who has the attributes of the Son of God and the Divine Mother, he will surely self-destruct. The Saint Destroyer forcefully destabilizes the balance that Divine Mother's Worship gives that allows Acqua of the Back to use his Saint powers beyond normal Saints, that is how he can be beaten. Acqua of the Back smiles at this revelation, continuing his assault on Kanzaki. She strikes at the buildings with her wires around Acqua of the Back, and there he discovers that she has formed from the rubble, a crown of thorns, an iron picket, and a large cross, the method of execution of the Son of God, the weakness that all Saints inherit. Acqua of the Back begins to waver, for surely, though to an ordinary Saint, this method won't do much effect, to someone like Acqua of the Back it spells out his execution. Itsuwa comes close again with her spear, symbolizing the Spear of Longinus that pierced the Son of God. However, Acqua of the Back isn't numbered yet. Impressed by their feat, he jumps again and uses the artificial moon of School District 22 to symbolize the attribute he is aligned to. He falls back the earth with his rod aiming at them, exuding a tremendous magical power. In danger, Kanzaki prepares to defend against Acqua of the Back, collecting all the symbols throughout history, forming this spell at the last second, turning Kanzaki Kaori into a shield.

Acqua of the Back strikes down, but to everyone surprise, the place where they're standing remains intact. Acqua of the Back's spell was destroyed by the arrival of Kamijou Touma, holding unto the rod that was fused with magic a moment ago. Touma falls down and seals Acqua of the Back. In his shock, Acqua of the Back could not see Kanzaki Kaori charging towards him, holding his rod and shoulder. With his movements sealed by Kanzaki, and magic negated by Touma as he tries to shake her off, Itsuwa charges towards him with her spear.

Acqua of the Back accepts the fate and determines to face it to the end. There, the spear strikes him, piercing his abdomen to the back. Crosses of light appear behind Acqua of the Back, intersecting in the middle, causing Acqua of the Back to fly back due to the impact. They later witness Acqua of the Back being propelled into the artificial lake of the 5th level, and then self-destructing, utterly destroying and boiling the lake into a pillar of steam.

Aftermath
Itsuwa is by Touma's side after the battle. Touma awake to find her. She tells Touma that it is only normal for him to be unable to move after all he had went through. Here, Itsuwa says that they Aqua of the Back is still alive and retreated, as well as being no one dead in the Amakusa or even among civilians. Touma then shows his nonchalance regarding the matter, saying that it was amazing that they've managed to beat him, a member of God's Right Seat and a Saint. Itsuwa is surprised by this from someone who contributed the most, despite her being the key to his defeat using the Saint Destroyer, as they have not only beaten him but did not have a single casualty. Touma tries to get out of bed again, needing to go to school, but Itsuwa stops him pushing down and making her face close to his, making her blush. Suddenly, Index come around, angry at him for having Itsuwa sit beside him instead of her as well as not yet apologizing for leaving the hospital on his own. Itsuwa joins the fray, asking why he ran over to where Acqua of the Back despite his condition. Hearing Acqua of the Back's name, Index becomes even angrier at Touma for facing such a powerful foe without consulting her for help. Needless to say, Touma is distraught over their double team.

Outside, Kanzaki is confused on how she should thank Touma. She was going to before but when she found out that Itsuwa and Index was in Touma's room, she lost her nerve. She becomes worried as she has to return to London the next day. Tsuchimikado Motoharu arrives and correctly deduces that she is unable to go to Touma's room because she is too embarrassed as Itsuwa and Index are there, with Kanzaki opting to wait for another time to thank Touma.

Motoharu then brings up the fallen angel maid costume, to which Kanzaki says she doesn't have it and it was too embarrassing to bring along and wear anyway. Motoharu, ever prepared, shows her the new fallen angel ero maid costume. From there Motoharu relentlessly teases her on how normal methods of thanking Touma is not enough, telling her to use her breasts to thank Touma, and on how Itsuwa would be willing to use the costume given the chance. Although she is so overwhelmed on what to do to the point of punching the floor, Kanzaki finally acquiesces and takes the costume. Ten minutes later, Kanzaki punches Motoharu in the face and barges into Touma's room, where he sees his "third angel" after Misha Kreutzev and Kazakiri Hyouka, though he is referred to as feeling fear upon seeing it from that onward.

Meanwhile, Aleister Crowley is inside his Windowless Building. Here, he is a bit bothered by the fact that due to Acqua of the Back's attack earlier, his UNDER_LINE surveillance network has collapsed. Here, he is shown a report regarding Kamijou Touma whose Imagine Breaker has a 98% chance of affecting the plan. There, alongside the number 1 of Academy City, Accelerator, the main cogs for his main plan is under way.

Destruction of St. Peter's Basilica
The Pope later hears about the defeat of Acqua of the Back, and becomes furious, knowing that there are two implications of his defeat: for one, it was a major blow to their fighting strength, and for another the enemy had assumed superior fighting strength for themselves. The other being Kamijou Touma who had many people whom spontaneously gathered around him for the sole purpose of protecting him, this circle of protective friends was his greatest strength, not Imagine Breaker itself. He is then surprised by the sudden arrival Fiamma of the Right who is unconcerned about the defeat of Acqua of the Back. He then states that their next plan is to attack England but, not for them being allied with Academy City but for something inside England. The Pope is then attacked by Fiamma of the Right, leading him to curse him for his plans to attack England would surely lead to a war in Europe.

The Pope then attacks Fiamma of the Right, to which he easily dispels and reveals the Holy Right, the power that creates all miracles and destroy all evil, though as he is still human he cannot utilize its full potential, destroying a part of the St. Peter's Basilica in the process. The Pope realizes that he needs both the materials and knowledge to create a spell that would enable Fiamma of the Right to master the Holy Right: Imagine Breaker and the Index Librorum Prohibotorum. Here, he realizes that Fiamma of the Right wants to create chaos in England in order to lure Index there. Knowing this, the Pope tries to resist again, but he is easily beaten by Fiamma of the Right who uses a power that could've destroyed the entire Vatican if not for the Pope diverting the destructive to himself, though the last attack finally razes the entirety of the basilica into rubble, Fiamma of the Right is impressed by this, though later leaves to set his plan in motion.

Vento of the Front later arrives and is informed by the Pope that Fiamma of the Right intends to go England, though she has no intention going there and wants to stop Fiamma of the Right's crazy plan in her own way. News of the destruction of the basilica and the in-fighting in the Roman Catholic Church reaches Laura Stuart as well.

Effects and future implications
With that, the Amakusa Christian Church becomes whole again, with Kanzaki Kaori once again controlling the faction.

Fiamma of the Right's plans later go into fruition, as the Eurotunnel bombing takes place under the manipulations of Fiamma of the Right, which forces Index and Touma to get to England to aid in the investigation, this and his intentions of attacking England is directly responsible for much of the next arc's conflict: the British Halloween.

Moreover, Fiamma of the Right later gains almost complete control of all the forces of Roman Catholic Church and the Russian Orthodox Church, a point that annoys Vento of the Front as she and even Acqua of the Back has no authority over the forces at all. Although, Acqua of the Back lost much of his power after his defeat, he still manages to survive and retreat back to England, just in time for the British Halloween. Sometime before making his public return, he secretly communicates with Vento of the Front, commenting on the position of England and France after the Eurotunnel bombing, and its political implications. Vento of the Front tells Acqua of the Back that she will not be joining him in England and instead "shock Fiamma a lot more" indicating her later actions in Russia.

The arc would mark the changing of Mikoto's role in the story as introduced in the previous one where she discovers Touma's memory loss. Being at this point the only one of Touma's friends that knows that he has lost his memories puts her in a unique relationship with him, coupled with her telling him to rely on her or ask for her help, as well as her realization of her own feelings for Touma, would later culminate in her attempting to aid him in his adventures. Although Mikoto would try to follow Touma in order to help him in some of his adventures, he himself would not ask for her help (along with Index) until a couple of months and a war later during the GREMLIN invasion of Tokyo.