It is the last day of summer vacation and Touma struggles with his homework, something that he has not touched at all. Meanwhile, Misaka Mikoto, going about her daily life as usual, leaving the dormitory, browsing in a convenience store, the usual routine, however, this time a young man is waiting for her. He is the grandson of the director of Tokiwadai Middle School, Unabara Mitsuki, a young man who has been following her around recently. As she is unable to shoo him away, she catches up to Touma in order to pretend to be her lover for the day. The plan is for Mitsuki to give up on her when he catches up to the fact that she seems to be dating. But their behavior is contrary to what lovers do and they gave up in the middle of their ruse. As an apology to Touma, Mikoto goes into a store to treat him. As Touma waits for her, Mitsuki appears before him. However, Touma then sees someone who looks the same as Mitsuki enter the store seemingly chasing after Mikoto.
Nighttime in the Kamijou Residence, a drop of blood fall on the cold bathroom floor as Touma awakens from his sleep in the bathtub due to a sudden nosebleed. Touma goes outside of the bathroom holding his nose, trying to find tissue, during which he states that the reason why he is sleeping in the bathroom is because of Index has taken over his head. Looking at her, Touma blushes at her sleeping figure which worsens his nosebleed. Touma eventually finds the tissue box after stepping on it, accidentally waking Index up. Seeing his expression, Index asks him what he is doing all the way from his bathroom. Touma hesitates in answering that he was simply there to clean his nosebleed, as he was completely embarrassed by the sight of Index's appearance (apparently wearing nothing but Touma's uniform that does not fit her), which makes his nosebleed worse than before. Index asks again if Touma did not feel anything when he saw her sleeping figure, to which Touma confirms that he felt nothing. Angered by this, Index bites Touma's head citing that it is an insult to her being a girl; Touma tries to retract his statement, though Index states it's too late and keeps biting Touma.
However, after falling down, Touma realizes that he has roughly 24 hours before summer vacation left to finish his unfinished homework. Touma inevitably bleeds profusely from Index's biting as he ponders this, and faints while lamenting his own misfortune.
It is August 31, during the morning a few minutes after breakfast in the Tokiwadai Dormitory. Misaka Mikoto is enjoying her tea when Tsuchimikado Maika calls out to her. Mikoto responds and reminds her that Maika is a maid apprentice, and shouldn't be talking as an equal to her. Maika asks Mikoto for a favor, wanting her to buy Maika a manga if Mikoto is going to the convenience store. Maika states that she prefers manga that isn't rated 18 and above, but still somewhat very steamy. Mikoto asks if Maika likes Boys Love kind of manga, to which Maika replies that Mikoto should know what she likes and then states that she loves the genre where brother and sister falls into a romantic yet troubled relationship. Mikoto concedes and leaves the dormitory.
While waiting on the sidewalk, a young man approaches and greets Mikoto, which surprises her; he asks Mikoto if he may accompany to wherever she is going. Mikoto says that his name is Unabara Mitsuki, and hesitates to answer his inquiry. Seeing this, Mitsuki asks if Mikoto is not feeling well, to which Mikoto responds that she is fine. Mikoto states in an inner monologue that Mitsuki has been following her every day of the week. Mitsuki then tells Mikoto that he knows a seafood restaurant, to which she comments to herself that asking someone to a meal right after breakfast doesn't make any sense. Mikoto then tells Mitsuki that she has something to do, and as such cannot accept his offer. However, Mitsuki says he will come with her, hearing his reply, Mikoto says that she's thinking about going to the lingerie section of a mall, a place where men aren't exactly comfortable. However, he still insists on going with her, much to her dismay.
Thinking of a way to get rid of Mitsuki, Mikoto notices the voice of a certain person. It's Touma walking with Motoharu and Aogami, begging them to help him in his unfinished homework, though it quickly deteriorates into a conversation regarding their preferred girl types, which annoys Touma. Seeing him, Mikoto decides that she can get rid of Mitsuki if she pretends that she is waiting for Touma. With one gulp, Mikoto runs towards Touma, pretending to apologize to him.
Unfortunately, Touma does not notice her and walks on with his friends. Angered at this, Mikoto tackles Touma to the ground. Touma looks up to see that Mikoto was the one who knocked him down; reddened by her embarrassment, Mikoto pleads to Touma play along. With that, Mikoto helps Touma up to his feet, apologizing to him for being late and promises to treat him as a way to make amends, which visibly shocks Motoharu and Aogami, disappoints Mitsuki, and absolutely confounds Touma. However, stirs of gossip by the girls of Tokiwadai start echoing through the halls of the building, as they watch the spectacle of Tokiwadai's Ace outside, impressing the Dorm Manager for Mikoto's brazen behavior, and distressing a present Kuroko. This event forces Mikoto to runaway bringing Touma with her.
While catching their breaths, Touma demands Mikoto why she dragged him with her, to which Mikoto tells him to shut up as she can't through to what just happened. They scene cuts to Touma and Mikoto enjoying hotdog sandwiches, where Touma discovers that Mitsuki has been stalking her everyday for the past week, and cannot simply get rid of him like she usually does because he is the grandson of her school's superintendent, and is the reason why he is forcing Touma to pretend as her boyfriend (and he just happened to walk by). Touma says that there is no reason to pretend anymore as she already got away from him, though Mikoto comments that she's going to use this opportunity to make people see them as a couple all day, as so Mitsuki will get discouraged and would stop harassing her; though Touma complains. Mikoto asks Touma for his thoughts regarding her plan, though starts off by advising Mikoto to wipe off the mustard on her nose, much to her embarrassment. Mikoto tries to wipe it off but ends up hurting herself in the process; seeing this, Touma checks if she is okay, to which an embarrassed and pained Mikoto confirms it. However, Mikoto notices that her sandwich and Touma's sandwich are next to each other and can't tell them apart, though Touma is apathetic towards this as he explains they both ordered the same thing, Mikoto insists that she try to figure which sandwich is hers with little success, as she is unable to figure which is which. Touma begins to eat the sandwich that Mikoto handed to him, which makes Mikoto's face turn red and puts her to silence. Touma wonders why she suddenly becomes so quiet, to which Mikoto replies that it's nothing and begins to nibble on her sandwich.
Their conversation changes on how there are only 10 of Mikoto's clones left in Academy City, and the rest went on to institutions outside the city for adjustments. The news pleases Touma, though Mikoto comments on what they're talking about is different to what couples usually talk about. Touma ignores her and takes out his worksheet for his homework; seeing this, Mikoto calls him out on his behavior. Touma explains that he has yet to finish his summer homework, and has little time for him to finish it. Hearing this, Mikoto becomes interested in Touma's homework and begins to answer the questions with ease on his worksheet, dispiriting Touma as he cannot solve the questions himself. Pleased with this, Mikoto offers to buy drinks for her and Touma, though Touma offers to buy them instead, Mikoto insists that she goes.
While waiting for Mikoto, Touma sees Mitsuki helping a child get his dog back. Mitsuki notices him as well and they talk. Mitsuki asks if Touma is Mikoto's friend, to which Touma replies if it bothers him. Mitsuki confirms this because Touma is close to Mikoto, a girl Mitsuki likes. They continue their conversation regarding Mikoto's personality, while Mitsuki answers Touma's homework for him. However, Mikoto arrives with the drinks, and asks Touma to come with her for a moment.
Mikoto questions Touma on how he is trying to become friends with Mitsuki; Mikoto says that Touma is supposed to help her keep Mitsuki away as his "boyfriend character". Touma however, refuse to follow her plans, and refutes her statement by saying that Mitsuki is a nice guy, and that he is serious about Mikoto; Touma asks Mikoto on what exactly she doesn't like about Mitsuki, and says that there is something wrong with her going out with someone she doesn't like, which refers to him. Defeated, Mikoto agrees to stop pretending to be a couple with Touma, and tells him that she'll treat him to some food before they leave.
Touma waits for Mikoto to return with the food from a burger joint, when Touma meets up with Mitsuki again. They converse on how Mikoto is struggling inside the establishment, however, when Touma turns to look, he sees someone who looks like Mitsuki frantically enter the building. Confused, Touma asks Mitsuki if he has a brother as he had just saw someone that looks like him enter the establishment; Mitsuki denies this.
Meanwhile in the store, the other Mitsuki appears before Mikoto, wearing bandages on his right arm; he falls down and tells Mikoto to run. Back with Touma and the other Mitsuki, he tells Touma that certain espers can change the appearance of their bodies, and hypothesize that the other Mitsuki Touma is referring to maybe pretending to be him to get close to Mikoto. Mitsuki then asks Touma to go check on Mikoto's safety, as he just angered Mikoto merely moments ago. However, after turning his back on Mitsuki, he suddenly feels being hit hard on the back.
Touma discovers that the one he is talking to is the impostor. The impostor bemoans on how his planned did not go well as the original Mitsuki has showed up. He takes out a shiny black dagger, and reflects light from it which destroys a car. Realizing that he's a magician, Touma makes a run for it as the impostor attacks again.
Back in the Kamijou Residence, Index is watching Magical Powered Kanamin and is intrigued by it. However, she receives a phonecall from Touma who is running away from the fake Mitsuki. In the phonecall, Touma asks if she is alright as he is being chased by a magician, Index confirms that she is alright, and asks a few things to help her identify the magician attacking Touma. Index states that the weapon the impostor is carrying is made out of obsidian, and is probably related to the lance of Tlahuizcalpantecuhtli – an ancient Aztec deity – said to kill anything that absorbs the light of Venus. Desperate, Touma pleads to Index to give him an easy hint on how to beat the impostor; Index answers that the lance is a replica, and can be destroyed by Touma's right hand. Touma asks another question regarding Aztec magic, regarding their capability to change appearance; Index states that Aztec priests flay the skin of their sacrifices and wear them, though if it's a disguise they'll only need to cut a little part of the skin and make it into a charm, as so they can mimic a person's appearance.
Touma then cuts-off his conversation with Index after hearing enough, and end up running into an abandoned construction site. Here, he is cornered by the impostor who tries to use the obsidian dagger on him. Thinking quickly, Touma grabs a shovel full of cement and throws it towards him, temporarily impairing the impostor's vision. However, he nearly gets hit by the shovel itself after Touma throws it at him, which Touma used quite successfully to allow him to come near the impostor and force him into close combat. The wind blows the dust away, which gives the impostor an opportunity to use his dagger; however it does not work as cement dust covers the dagger, preventing light to be reflected from it. With the impostor distracted, Touma uses the opportunity to punch him in the face, cracking his mask.
With the impostor on the ground, Touma demands answers from him. He asks him why he pretended to be Mitsuki so she can get close to Mikoto, as she is not involved with the magic side. Mitsuki tells Touma that he is in possession of the holder of the 103,000 grimoires, as well as magicians and espers as his allies, a faction of sorts. The forming Kamijou faction, according to impostor, is a very unstable yet influential power that threatens the balance of both sides. Mitsuki tells Touma that he intends to destroy the faction within by pretending to be someone close to its members. However, the impostor tells him his cover is blown and needs a new face a disguise. Suddenly he moves to take back his obsidian on the ground, hinting that he should steal Touma's face instead, wipes the dust off of it and attacks again.
Touma dodges the light, which makes the impostor hit the steel beams of the unfinished building. With the impostor distracted, Touma punches the impostor's dagger and destroys it. The two fight it out using their fists alone, while the building falls apart around them. Suddenly, a steel beam falls between them, temporarily halting the battle. In the lull, Touma tells the impostor that he thought he could've been friends with him, though the impostor does not feel the same. Touma asks him if the talk they had regarding Mikoto was a lie as a well. The impostor responds by asking if there is anything wrong with lying to protect Mikoto; the impostor states that he didn't want this to happen as well but his superiors have insisted that Touma and his faction is to dangerous. Touma realizes that the impostor truly likes Mikoto, which the impostor confirms; however, he is now Touma's enemy. While the unfinished building is falling apart, Touma states that he will break his illusion apart.
They continue their brawl while metal beams fall from the structure, however, the entire building collapses, with one of the steel beams hitting the impostor. After the collapse, Touma is unscathed and goes to see the impostor's condition, seemingly brought into unconsciousness by the impact of the beams on his body. However, he wakes up and asks Touma if he has lost, to which Touma replies he doesn't know. The impostor tells Touma that this will not be the only assault on him, and says that Touma and Mikoto will continue to be targeted. As such, the impostor asks Touma if he could protect her; to run to her wherever and whenever she may be. Touma replies to his question and smiles, to which the impostor states that Touma's answer is the worst one he could expect.
Mikoto overhearing their conversation, is visibly reddened by Touma's answer and tries to shake it off. She says to herself that he is the kind of person who would say such irresponsible things, and denies to herself that she is something special to him. She rests her on a metal beam and looks at the sky, stating to herself that even though she is misunderstanding him, his words are very misleading.
- Touma defeats Etzali, and promises him to protect Mikoto and her world.
- Unabara Mitsuki escapes Etzali's grasp.
By order of appearance
- Kamijou Touma
- Index Librorum Prohibitorum
- Misaka Mikoto
- Tsuchimikado Maika
- Aogami Pierce
- Tsuchimikado Motoharu
- Tokiwadai Dorm Supervisor
- Shirai Kuroko
- Imagine Breaker - Kamijou Touma, Level 0
- Electromaster - Misaka Mikoto, Level 5
- Esper power to change appearance (referenced)
- Appearance Borrowing - Etzali
- Spear of Tlahuizcalpantecuhtli - Etzali
- When Mikoto tackled Touma to the ground, he called her Biribiri.
- The episode features the Magical Powered Kanamin show for the first time.
- Etzali (the impostor) is unnamed throughout the entire episode.
- This would be the first time the Kamijou Faction is referred to by a character.
- This episode marks the first appearance of Tokiwadai Dorm Supervisor's face in Toaru Majitsu no Index anime. Previously she appears in the 11th episode, but only her legs were shown, and her voice was heard.
- The Sisters being in body readjustments is referred to during this episode.
- Touma references the common anime/manga element of having a nosebleed when characters get into erotic situations.
- Mikoto references Boys' Love, which refers to fictional media that focus on the erotic or romantic relationships between males.
- Aogami references several female character archtypes in anime and manga during his conversation with Touma and Motoharu, such as the shrine maidens, the tsundere and the yandere. Strangely, Aogami also references shotacon, which refers to the attraction to young boys.
- MocRonall's maybe a reference to McDonald's.
- Tlahuizcalpantecuhtli is a dangerous and malevolent ancient Aztec deity, and is considered the god of the planet Venus.
- Obsidian is a natural occurring volcanic glass formed as an extrusive igneous rock. Obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points and blades, most notably, ancient Mesoamerican civilizations such as the Aztecs.
- Magical Powered Kanamin maybe a referenced to the magical girl subgenre in anime and manga.
- The Tokiwadai Dorm Supervisor in this episode, features a drastically different character design from the same character in the Toaru Majutsu no Index manga, as well as Toaru Kagaku no Railgun manga and anime.
- Who are the impostor's superiors?
- What happened to the impostor after this episode?
- What was the answer Touma gave to the Aztec magician?
- Maika: "I love the genre where a brother and a sister fall into a deep and muddy romance." -said to Mikoto, regarding her favorite type of manga.
- Tokiwadai Dorm Supervisor: "How bold of her... to be rendezvousing in front of the dormitory." -regarding Mikoto meeting with Touma in front of the Tokiwadai Dormitory.
- Touma: "Then that leaves me no choice. I will kill that illusion of yours!" -said to the impostor.
- Etzali: "Could you protect her? Protect Misaka-san? Run to her wherever and whenever... Promise me you'll protect her?" -said to Touma.
- Mikoto: "He's the kind of person who would say such irresponsible things! It's not like I'm special to him or anything! I know I'm misunderstanding, but... it's so misleading!" -regarding Touma's reply to the impostor.