Naota and Mamimi are under a bridge by the river in the town of Mabase. Mamimi is monologuing about how to swing a bat in a game and says, “What are you talking about? Are you talking about a video game?” Mamimi asks what he is doing and Naota says he is doing his homework. When she asks why doesn’t he do it at home, he replies, “It’s not cool.” When Mamimi says he can do her homework if he wants to, he looks at her and says her hands are upside down, referring to the baseball bat. Mamimi says, “You should play baseball too, Takkun. Why do you always carry around a bat?”, and he counters with, “Why do you always hang around here by the river?” Mamimi says, “Because you know, huh, now why was it?", so Naota replies, “You're in a good mood”. Mamimi drops the bat and begins caressing Naota. When he says she smells of cigarettes, she denies that she is a smoker. As she caresses him and nibbles his earlobes, Naota says, “Mamimi, why do you always do this?”, and she responds, “If I don’t do it, I’ll overflow”. When Naota asks what that means, Mamimi says, “Probably something amazing,” and collapses on him.
After the monologue, Naota and Mamimi go to the beverage machine, and just as Naota is about to pick his drink, Mamimi orders one before him. He tells her that she knows he doesn’t like sour drinks. She notices a mark on his neck, which he quickly and embarrassingly covers up, so she suggests a band-aid. She gives him the rest of the drink and walks away. He throws it on the ground, and says, “You know what? My brother in the U.S...”, which stops Mamimi in her tracks, but before he can finish, Haruko drives at high speed on her Vespa towards them. Haruko screams, “Alright, lunchtime!”, and runs right into Naota with her Vespa. As he lays unconscious, Haruko revives him with CPR. As he gets up, she hits him on the head with her Rickenbacker, trying to open his N.O. channel. She wonders why anything isn’t coming out and suggests hitting him again, but Mamimi stops her. Naota shouts in pain, “What did you do to me?!” and Haruko says she just did CPR. Naota states that wasn’t what he meant and begins spitting in disgust. Mamimi questions if Haruko put her tongue in his mouth. Haruko gets back on her Vespa, says Naota is useless, and he yells, “What are you talking about?!”, but she drives off. Naota says Haruko is a stupid adult who doesn’t know how to grow up.
At night, back at Naota’s home in his room, he says to himself his bump isn’t normal and he can’t go to the hospital looking the way he does. He tries pushing the horn-like bump back into his head, which works at first, but it pops back out, which makes him reel in pain. The next day at his school, Naota wears at bandage on his head where the horn is. Ninamori tells him it looks bizarre and assumes he is hiding something, so Naota says, “Don’t be such a brat.” Gaku asks Naota if he heard about the Vespa woman. Masashi and Gaku says she has a guitar, motorbike, and a girl in another class had her super spicy curry bread stolen by her. This makes Naota realize it was the pink-haired woman from yesterday who is the Vespa driver. Gaku goes on to say that she stings you and leaves a demon mark that shows you’ve done naughty things which never goes away. Ninamori teases Naota about the mark on his forehead and Gaku says it’s the mark of the demon, so they ask to see it. Naota says there isn’t anything to see and tells his friends to cut it out, as Gaku yells he is pervert.
After school, Naota is on his way to the hospital and didn’t want to think of the Vespa woman. He is waiting for the train to pass so he can continue, but Haruko appears on her Vespa right behind him. She is observing his head, to make sure it’s a proper head for N.O., and asks what happened to him yesterday after she left and if it was weird. Naota claims he doesn’t know what she’s talking about, but Haruko asks if there is a pimple on his forehead and if he’s hiding something. When she looks away, he runs across the train track, with the train blocking his movements from Haruko. He arrives at the hospital so a doctor can check his bump. He is laying on a table, while a nurse diagnoses him. She states he has Flictonic Clipple Webber Syndrome, an adolescent psychological skin hardening syndrome, where children grow horns from trying too hard, but Naota accuses her of lying, as he has never heard of it before. She admits to lying and reveals herself to be Haruko, trying to see what’s underneath the band-aid. She tries to hit him again with her guitar, but Naota manages to sneak out.
Back at home, he is on the phone with Mamimi. She asks him something about photos, but he doesn’t want to see them so he says he is busy, doesn’t care, she should go out with her friends, and hangs up the phone. He lays down on his bed and falls asleep. He dreams of his brother batting, amazed by his swing. He wakes up and his father calls him downstairs to eat dinner. When enters the kitchen, a manga-like scenario occurs as Naota is surprised and disturbed to see Haruko in his home. His father introduces her and says they met because he was also hit by her Vespa, and tells Naota she is their new live-in housekeeper, which leaves him dumbfounded. Haruko claims Naota was making out with a high-schooler. His father asks who he is fooling around with and Naota quickly states it was him and Mamimi by the river. His grandfather says, “I bet she does it like a weasel, she’s cursed I tell ya.” Kamon claims since Naota’s brother is away, she’s “sinking her fangs into you now, fondling around, fooling around, Fooly Cooly!” Kamon says, “What’s Fooly Cooly?” and Naota yells, “How should I know? I’m still in grade school. You think I’m that crazy?” Kamon thinks it’s like Gundam, basically a robot anime. Haruko says she and Naota are already in a mouth-to-mouth relationship, which infuriates Naota. Kamon states in a jealous way they’re already living in the same house and “Fooly Coolying,” but Naota says, “Who cares if it’s Fooly or Cooly?!” Kamon believes he dreams of it and is hiding something under the band-aid and tries to take it off.
Naota leaves to take a bath, in anger. During his bath, he could hear them and said sullenly, “They’re in a good mood.” He goes to his room and hears Haruko, who was just talking to Fraternity, and gloomily tells her to ask before she enters. When Haruko states she will sleep on the top bunk of his bunk bed, since he’s on the bottom, Naota asks about who she is and what her intentions are. Haruko simply says she’s a wandering housekeeper, but Naota believes she is lying and asks for the truth, so she says she is an alien. Unbelieving, Naota says, “Yeah right, and this afternoon you were a nurse.” Haruko asks what’s underneath his band-aid, which prompts Naota to call her freaky. She continues to ask what’s it like, but Naota says he doesn’t know. Haruko tells him he is lying, and because he saw it he believes something strange is happening. Haruko still asks what’s under the band-aid and Naota accuses her of causing it. Haruko tells him, “You should know about your own head,” so Naota says defensively, “Because you hit me there.” Haruko yells, “Let’s see it!” and moves to remove the band-aid from his head, but Naota swipes her hand away and yells, “You came here with my father, why don’t you go stay with him?!” He calms down and asks what she wants. Haruko says he is the one she saw first, which makes Naota blush. Naota changes the subject and says no one else can sleep on the top bunk since it belongs to his brother. He lays on his bottom bed and says, “That’s the way it is.” Haruko asks how old Naota’s brother is and where he is. Naota answers he is in America. When Haruko asks why he is in America, Naota answers that his brother plays baseball. Haruko says he must be a good player and lays next to Naota. This makes Naota blush and embarrassingly asks why she’s sleeping next to him, to which she states the top bed belongs to his brother and his bed is the only place she can sleep. Naota leaves his room and goes downstairs with his pillow, believing she is playing with his head.
His father calls him and says they need to talk. Naota asks about what and Kamon says it’s about Haruko. Kamon says he believes Naota is against her staying in their home, but Naota states his father can do whatever he wants. Kamon smiles creepily, which leaves Naota disturbed. Kamon says he named Naota for “honesty” and shows pictures of Naota being run over. Naota immediately asks if Mamimi came to their home and Kamon says she asked for the day-old bread they had on sale. Kamon asks if Mamimi’s family is poor and Naota runs outside after her.
He goes to the bridge they often hang around and finds Mamimi smoking a cigarette. Mamimi is cradling the bread and says he gave her a lot, referring to Tasuku. Naota asks if his brother sent her a postcard or made any form of contact with her, and then asks how much she likes him. She feels the bread and says it’s hard, so Naota reminds her she saw the sign at his family’s bakery and it’s old. She puts it down and says, “Watermelon, or like a panda with a mean face, or like sandals with pressure points drawn on them, or the smell of a blackboard eraser, or a Sunday morning where you wake up and it’s raining. Well, I like him more than hard bread.” Naota says she should stop because his brother has an American girlfriend. When Mamimi grabs her head as if in pain, Naota quickly asks if she is alright. Mamimi says she is going to overflow, but Naota doesn’t know what she means. She collapses and suddenly his N.O. activates, his band-aid coming off as his horn extends. A red Canti and robot arm come out of his head, fighting. In a daze, Naota unknowingly snags his shirt on Canti and is caught up in the two robots' battle. He is concerned for Mamimi, but tries to free himself from Canti. He sees Haruko arrive swinging her Rickenbacker into Canti’s head, damaging him and turning him from red to a blue-greenish color. He shouts in amazement, but he believes shouldn’t have. In a flash, Haruko briefly looked like his brother to him. Haruko is trying to find someone, presumably Atomsk, while Naota is still stuck to Canti.
The next morning, Naota’s horn is gone and so is the band-aid. He believes it was nothing amazing, despite the surrealism of robots emerging from his head and fighting. Canti comes to his house and becomes his family’s servant. He puts his shoes on and runs to the bridge, finding Mamimi at the beverage machine. He is out of breath and Mamimi tells him he didn’t have to run the whole way, offering him some of her drink. He recognizes it as a sour drink, but despite his distaste for them, he drinks it anyway.
As Naota is leaving school, he sees Mamimi near the entrance and asks her why she came to his school. She notices his hat and asks what’s on his head, as Ninamori, Gaku and Masashi watch from afar. Back at Naota’s house, Haruko takes an X-ray of Naota’s skull and shows there is no brain on the picture. Naota angrily tells Haruko that she caused this, but Haruko says, “Don’t blame me if you go around losing things. Some kids are just freaks.” Naota counters, “Look who’s talking, Ms. Normal.” Haruko states she didn’t have a robot come out of her head then tells him another horn appeared on the back his head. Naota tells her the horns aren’t normal, reminding her the last one sprouted Canti from his head and is living with his family. He blames Haruko since he was run over by her bike, but she insists it’s a normal Vespa. Naota tells her she hasn’t taken the nurse outfit back and people think she’s his family’s actual nurse. When she reveals her identity as a First Class Space Patrol Officer, Naota believes it’s just a lame stage name and she’s older than 19. She smacks him with the end of her guitar, which gives him a bump on his forehead. He asks her why his cat Miyu Miyu got on her case yesterday and she says, “Ah...because I'm in trouble. Lots of trouble. Because of Takkun, they’re all on the case.” He asks what Canti is and how he can take X-rays. Haruko says it’s because Canti was made by Medical Mechanica and they make them in the factory shaped like an iron. Naota asks why it came out of his head and Haruko says that’s what she wants to know. She asks what his connection to Medical Mechanica is and repeatedly jabs him with her guitar, much to his discomfort.
When Naota notices Kamon using Canti to move boxes into his car, he yells at his father for using Canti outside and says the neighbors will see him. When Kamon claims Naota just wants Canti to himself since he brought him home, Naota argues about the weirdness of having a robot in their home and Kamon begins monologuing about what it means to have a robot in the house. Haruko asks what Kamon is talking about and Naota says Kamon wrote a book on the deep mysteries of Eva. Naota quickly turns his attention back to his father and yells that Canti was walking around freely yesterday. Haruko tells Naota there have been fires spreading around lately and Naota tells her one just happened yesterday.
Later that day, Naota goes under the bridge and finds Mamimi playing the game Fire Starter, with her feet in water. He throws her a pair of sandals and asks what she is doing. She says she was playing with Takkun and her shoes floated away. He is confused about who she is referring to, so she shows him a small black cat she named Takkun. He asks, “That’s his name too?” Mamimi confirms and asks if he has anything for the cat to eat. Naota asks what he could feed it and Mamimi says, “Not like chewing gum.” Naota questions if she saved the stray cat and Mamimi responds with, “I'm like a god, huh? A protector of the weak. The God of the Black Flames. You know, Lord Canti, chief.” Naota blushes from the way he sees her plump lips move. He asks if that’s the game where she burns down a city. Mamimi says it is and she’s addicted to playing it. Naota asks if she likes games and Mamimi offers to let him play, but Naota says he’s too busy. She complains that he is "always busy." Naota says there are people he has to take care of and leaves with a good-bye as he opens his umbrella for the rain.
After Haruko’s failed attempt to break into the Medical Mechanica plant resulted in her crashing her Vespa, Naota arrives to bring her a box of spare parts. The 'parts' are a toy robot which somehow turns into a motorcycle part when Haruko screws it in. Two police officers had arrived and failed to help, being distracted instead. Naota slams the box on the head of the younger officer who tries to sneak a peek at Haruko’s underwear, asking if he is even embarrassed acting that way. When the younger officer asks if Naota is Haruko’s brother, Naota says, “No, no relation. I'm from planet Earth.”, so the officer tells him to leave since he’s working and it isn’t a place for little kids. Haruko states the plant isn’t a place for people to work, since there’s no entrance or exit. She begins fixing her Vespa and tells the police that Naota is her guardian. The older officer asks Haruko if Naota is really her guardian and Naota says, “Yeah, I have custody. It's kind of unusual.” Haruko says, “So, you came for me, huh? The rain stopped. I'll give you a reward. It's even better than CPR.” This suggestive language confuses Naota.
As Haruko and Naota are riding on the Vespa during the sunset, she asks if he usually goes to the beach and he replies he doesn’t come in October, because it’s too cold. Haruko begins to scare Naota with her dangerous driving maneuvers, so he yells that her driving sucks, but Haruko counters she’s a very safe driver. She continues the dangerous moves, and Naota says he doesn’t like scary stuff and he isn’t wearing a helmet. Haruko remarks he is scared, even though his brain is missing.
Back at Naota’s house, it is revealed Mamimi followed Canti back to Naota. Naota worries if someone will see him walking around and Haruko says she saw him at a Seven-Eleven with a magazine called “Hustler”, so Naota questions when. Mamimi is sitting outside, drinking tea, and wishes she could have Canti at her house. Naota remembers Mamimi isn’t allowed to have pets at her house. Haruko asks Mamimi to come inside because it’s cold, but Mamimi says she’s okay outside. Naota says his grandfather doesn’t like her dating his brother and Haruko questions if that’s the reason he’s keeping her out of the house, but Naota recalls that Mamimi said she didn’t care. Canti returns in a flash of light, much to Mamimi’s delight, but the lights briefly scares Naota. Mamimi officially calls him “Lord Canti”. Haruko tells Canti he is late and snatches her bag out of his hand while Canti types something on his screen. Naota wonders why she is using him to run errands, as Haruko yells at Canti for not getting her orange juice with pulp, but notices another bag in his hand. Canti knocks on the door and Shigekuni comes out, much to Mamimi’s annoyance. Shigekuni takes the other bag from Canti and yells at him, like Haruko, for getting the wrong centerfold magazine and slams the door. Naota takes notice of his grandfather using Canti for small tasks as well. Haruko claims Canti is useless, but Naota quickly states she is the one broken. Mamimi tells Haruko she shouldn’t use Canti because he is a god. Haruko is confused until Naota tells her Mamimi got the idea from a video game. Mamimi says the black wings on him proves he is real, even though they are clearly costume pieces. Naota yells at Canti not to leave his home anymore or he will chain him down. Haruko shows Naota and Mamimi a photo of Canti flying by the Mabase bridge over the river in her magazine.
By the riverside, while Canti is looking for parts, Naota and his middle school friends come to see the remains of a burned down boathouse. They wonder who the arsonist is and mention a school that burned down when they were little. Naota reminds them they can still see part of it and it’s all ashes. They all notice a shoe floating down the river and see Mamimi wringing out her uniform by the water. Masashi mentions he saw other girls bullying her yesterday and that her uniform resembles one from Shinden High School. Naota begins to see a brief flashback and looks at Mamimi as she turns toward his direction.
That night, Naota and Mamimi are walking down the bridge. Mamimi is barefoot, dragging her bag, and Naota briefly looks at her with a sad, concerned look. He stops as Mamimi drops her Fire Starter game and lights a cigarette, continuing to walk. He sees another fire start in the distance and notices Mamimi is conveniently gone. He remembers Mamimi telling him that his brother rescued her during the elementary school fire, calling him a "protector of the weak like Canti," and remembers rumors of arson regarding that event that was never solved. It is heavily suggested Mamimi is an arsonist at this point, explaining her attachment to Tasuku.
Naota finds Mamimi with a circle of candles around her and on her head, praying to Canti in front of the ashes of the school he had been thinking about. She notices him and Naota tries to give back her game, but she says he can keep it. Naota notices the old elementary school still remains and Mamimi says she hated it. Naota sees Canti, who is still searching for parts, and the robot quickly hides. Mamimi says, “And so, I wished that it would go away. But it's useless. Even if you burn it down, the wreckage and the ash are still there. Look, Endsville is burning. Now, Lord Canti, bless me with your kiss. Your absolution, kiss me.” As she moves to kiss Canti, Naota sees how her lips look and begins to feel a strong emotion, which causes his N.O. channel to activate. A giant robot with just an extendable right hand comes out of Naota and flicks him away into an arriving Haruko. It easily overwhelms Canti, trips Mamimi, and electrocutes Haruko. Naota tries to push Mamimi out of the way before Canti falls on her. When this act causes Naota to remember Tasuku's similar action, Atomsk is triggered within Canti and knocks the robot away. Canti swallows Naota and their fusion recreates the previous red Canti. Mamimi is so upset that Canti presumably killed Naota, but stops when she sees the enhanced machine’s power as it easily overwhelms the robot. Haruko takes an opportunity to destroy it with her guitar, which causes a fiery explosion that completely engulfs the nearby area. Canti rubs Mamimi’s head and ejects Naota in a manner similar to defecation, turning blue again. Naota is initially dazed by the experience and gains a large bump on his forehead, but eventually remarks that all the ashes are blown away. Haruko tells them to hurry before the authorities arrive, but Naota and Mamimi are preoccupied with looking at each other. Naota decides in his head to stay by Mamimi’s side forever.
Naota is in bed and wakes up from his slumber, because of Haruko working on her Vespa. After he puts his clothes on, he goes to Haruko and calls her “housekeeper wannabe”, but she continues working on the Vespa. He tries to speak to her again by calling her “cleaning woman”, but she still doesn’t hear him. For a third time he calls her “bottom beater” and she still continues working. Annoyed, Naota calls her an “evil alien”, which strangely gets Haruko’s attention. He asks her to clean up the mess she made in his room.