Index Arc

The Index Arc is the first major story arc introduced in the Toaru Majutsu no Index series. The arc introduces Kamijou Touma, a young man with the mysterious power of the Imagine Breaker, who meets a mysterious nun by the name of Index which pulls him into the mysterious world of magic. This arc occurs near the end of the Level Upper Arc in the Toaru Kagaku no Railgun sidestory.

Index Librorum Prohibitorum of Necessarius

 * Main article: Index

Index, a member of the Church of Necessary Evil is a girl who is capable of perfectly memorizing anything, and along with her ability to resist the corrupting powers of grimoires was used, or rather, allowed herself to be used as the container of 103,000 volumes of magical texts by the church. However, with such power in Index's disposal, the church was wary of her betraying the church and using the grimoires against them. Because of this, the church implanted a magical rune inside of her mouth that would kill her if her memories were not erased every year.

The Archbishop of Necessarius - Laura Stuart, lied to her subordinates of the true state of Index, which includes Stiyl Magnus and Kanzaki Kaori who were friends of Index. This forced both Stiyl and Kaori to look like enemies to Index, as so she would not form memories with them as so to not hurt her feelings when they eventually have to erase her memories. This continued on for quite some time, until Index somehow managed to end up in Japan, and easily make her way into Academy City.

July 19
Kamijou Touma - a level 0 student in Academy City is being chased around by thugs around the city, because he provoked them as so he can save them from Misaka Mikoto, a level 5 esper. However, his plans fail as Mikoto has caught up with him and has knocked-out the the thugs. Interested in testing his mettle, Mikoto challenged and attacked him, however he is unharmed as he used his ability - Imagine Breaker. Surprised that a level 0 like him can stop an attack from a level 5 like her, he continues to attack him using a powerful electric attack that caused a power outage on the city. Touma, however was able to escape and returns to his dorm.

At the same time, Index is moving from rooftop-to-rooftop to escape the magicians that are chasing her (Stiyl and Kaori), she however, slips and lands on the balcony of Kamijou Touma.

Meeting with Index
Kamiijou Touma meets Index the next morning and offers her the now-spoiled food (due to the power outage) from his refrigerator. Here, she tells him that she is being chased by magicians for holding 103,000 volumes of grimoires; Touma however is highly skeptical of the existence of magic, and tells her the existence of esper powers and his own, the Imagine Breaker. He tests it out on her magical habit, the Walking Church and destroys it, turning her nude for a few moments and biting him. She later fixes it, though it has lost it's magical powers of defense and leaves Touma's apartment after explaining that his power also removes his luck, much to his concern; her parting words to him is that if he was willing to follow him to the depths of hell. However, he knew that she would return as she had left her headdress.

Touma vs. Stiyl Magnus
After running into Misaka Mikoto once again, and taking care of the usual predicament of her wanting to fight him. He discovers Index lying in her own pool of blood, as the result of the attack of magicians. A magician named Stiyl Magnus comes along and explains that Index contains the 103,000 volumes inside her head, and proceeds to fight Touma for him to let her go. Touma defeats Stiyl even after summoning Innocentius, because of a major flaw in the runes that he used, as the ink written on it can be easily removed with the apartment's sprinkler system that Touma utilized.

As Index was severely injured, and as Touma or any other person he knows are unable to do magic (espers using magic is dangerous), he brings her to Tsukuyomi Komoe's house. Where, with her assitance, Index was able to completely heal herself through the use of a complex spell.

July 21 to 24
The next day, Index wakes up while Komoe allows her and Touma to stay in here for a while. During this time Index explains to Touma that she is a member of the Church of Necessary Evil, Necessarius and it's origins and purpose, and that she was used as a vessel to store the grimoires. Meanwhile Stiyl and Kanzaki observe them, and are preparing to attack Touma.

A couple of days later, Index and Touma decide to go to a bathhouse together, however, due to a misunderstanding that Touma is angry at Index about her not having any memories since coming to Japan, she runs away from him. While trying to find her, Touma runs into a barrier that removes people except for himself, and discovers that it is the work of magicians.

Touma vs. Kanzaki Kaori
Touma discovers that the magician responsible is Kanzaki Kaori, a member of Necessarius; a fact that surprises Touma as he never thought that Index's own faction would attack her. During their confrontation, Touma learns that both Stiyl and Kaori are members of Necessarius, and are out to retrieve Index as to erase her memories again. Kaori explains that Index requires that her memories needed to be erased every year, as so she can live because she has can pefectly memorize anything and that her mind cannot get rid of the unnecessary memories. She also explains as to why they presented themselves as enemies to Index, as so she would not be hurt again when the time comes that her memories would be erased, as they were friends.

During the entire battle, Touma is unable land a single blow on Kaori, as she is not using magic in her attacks that are injuring him severely. Despite this he perseveres, and delivers a rebuttal towards her actions. He states, that they are just being selfish towards Index, as they did not take into account on what Index would feel if her friends would carry on the burden of trying to erase the memories as so that she can live. He goes on to state that regardless of Index losing her memories and forgetting the friendship between them, there is no stopping them in trying to become friends with her again regardless of what they had to do.

Interestingly, in the Light Novel Touma's determination shown in his words and actions shook Kankazi's mind to the point she was blown away and her nodachi flew out of her hands by Touma's fist, when he was injured to the point where his own fist would bleed profusely and cannot possbily physically harm anyone. This in later novels fuelled speculations and rumors in the Magic Side that Touma have enough power to match a Saint in combat.

He later collapses due to his injuries, and was carried back by Komoe to her apartment.

July 27 to 28
Touma wakes up three days later, and realizes that July 27 was the day that the magicians were planning on erasing her memories, and it became obvious that Index was as well deteriorating. Predictably, the magicians came to take Index. Touma in desperation, asks them if they can use the facilities of the Science side - Academy City, but is denied by them as there is no time, and would not allow science to tamper with her. Touma is then challenged by Stiyl to destroy the magic on his cross that would heal Index, if he is confident enough of keeping Index alive; although hesitant, Touma decided not to do it. Kaori, however - decided to give Touma a few minutes with Index as to say goodbye.

Touma later becomes suspicious of the explanation of the church on why Index needed her memories erased, realizing this - he calls Komoe and asks her what she knows. She reveals that even if someone was capable of perfectly memorizing anything, it does not imply that the brain would not be able to handle it, as it has several parts that govern certain functions, one of which is memory, she continues on that the human brain can store hundreds of years of memory without problems. Touma, knowing that the church lied tries to find some magical apparatus that is making Index sick; he discovers that there is a magical symbol stuck within the roof of her mouth and decides to dispel it. It works, however - it activates Index's Johann's Pen Mode, and makes Touma the target.

Touma vs. Johann's Pen Mode Index
Realizing Touma as a threat, Index - now possessed by Johann's Pen, uses St. George's Sanctuary to distort the fabric of space within Komoe's room as a defensive measure against him. Index then uses Dragon's Breath as an offensive spell against him, but he is able to block it with his right hand, however - he realizes that he cannot dispel it. Stiyl and Kaori, discovers the ruckus as well, here - Touma tells to the both of them that their church was lying to them all the time about Index. Both of them help Touma to bring her back to normal, Stiyl summons Innocentius to aid Touma in blocking the attack while Kaori manages to make Index lose her balance, making her fire Dragon's Breath towards the sky, destroying Komoe's roof and destroying an Academy City satellite orbiting space.

Now having an opening, Touma runs towards Index and destroys St. George's Sanctuary - restoring the fabric of space in the room to normalcy, however, a dangerous after-effect of Index's Dragon's Breath - Feather of Light hits Touma's head and making him collapse near Index.

Aftermath
Touma is sent by Kaori and Stiyl to a certain hospital where the Heaven Canceler works in, where he successfully revives Touma but is unable to restore his memories. Index is present in the facility, where the Heaven Canceler tells her the truth of Touma's condition. Index also reads Stiyl's letter to Touma, where it is stated that Touma is allowed to keep Index, as the church has realized that the truth has been revealed; however, he also states that they will be back after gathering the necessary materials.

Before visiting Touma's ward, Heaven Canceler warns Index that Touma will not recognize her. Even then she steels herself and enter Touma's room, predictably - Index realizes that Touma has no recollection of various events that occured, as well as his name, in the end - Index is in the verge of crying where she outright confesses to Touma that she loves him.

However, before she could cry, Touma laughs and tells her that he was joking and that his memories have been restored after using his right hand on his head. This made Index angry, punishing him by biting him several times and leaves. However, Heaven Canceler asks him if he judged correctly on not telling Index the truth, it is revealed that Touma actually did lose his memories and that the only thing that he seem to deem as fact, is the contents of the letter that Stiyl gave to Heaven Canceler for Touma.

Touma tells him that he lied because he did not want Index to cry. He further states that even if his memories are lost from his mind, they will remain in his heart.

Trivia

 * The Toaru Majutsu no Index storyline officially begins in this arc. This arc also begins near the end of the Level Upper Arc in the Toaru Kagaku no Railgun storyline.
 * In the Toaru Kagaku no Railgun manga, the blackout caused my Mikoto due to fighting Touma on a bridge after failing to get information on the Level Upper is the same event that appears in the beginning of Toaru Majutsu no Index. This event was completely changed in the anime adaptation as Mikoto caused the blackout while fighting Elder Sister, the anime still indicates that Mikoto fought Touma on a bridge which implies that in the anime's timeline, there were two blackouts that were caused by Mikoto.
 * Komoe later references the incident where the roof of her house got destroyed, in episode 17 of the Toaru Kagaku no Railgun anime to Tessou Tsuzuri.
 * Index is still the only girl in the series to have confessed to Touma of her feelings.