Makoto first sees Kotonoha on the train. After seeing Kotonoha's picture on Makoto's cellphone, Sekai decides to play cupid and match Makoto together with Kotonoha by having lunch together. The next day, Makoto confesses to Kotonoha, which she accepts. Later that afternoon, while Makoto was thanking Sekai for setting him and Kotonoha up, not knowing how he should thank her, Sekai kisses him, saying that it was enough as a reward, and runs inside the train shouting "good luck" to a dazed Makoto.

Makoto is caught reading a love guidebook while waiting for Kotonoha for their first date. They wander around doing various things. When they part, Makoto tries to kiss Kotonoha but the subway's door blocks them. Makoto tells Sekai how the day went and she calls him an idiot for what he did. She gives Makoto two movie tickets so he can go on another date with Kotonoha and entertain her this time. Makoto messes up again and the relationship between Kotonoha starts to shake. Sekai then tells Kotonaha that Makoto is trying hard and she should try to get to know him better. The episode culminates with a kiss between Makoto and Kotonoha, with Sekai watching in the shadows.

At school, Kotonoha tells Makoto that Kokoro wants to see him, and asked him if he can go to her house on Sunday. Makoto happily accepts the invitation. Later while they are taking the train home, Hikari sees the couple together and thinks Makoto is cheating on Sekai, and sends her a text message saying that her boyfriend is flirting with another girl. Sekai quickly explains that they are just friends and nothing else. On Sunday, after Makoto goes to Kotonoha's house, he tells Sekai on the phone that being with Kotonoha is tiring.

While Kotonoha is busy studying at her house, Kokoro asks if she has kissed Makoto yet. Kotonoha avoids answering, but doesn't deny it. At school, Sekai speaks with her friends about various things, all the while thinking only about the kiss she shared with Makoto. On the roof, Makoto and Kotonoha are sharing a deep kiss when Makoto suddenly gropes Kotonoha's breasts, causing her to become embarrassed. She talks to Sekai later that night, Sekai explaining that lovers do this all the time, but Makoto is moving too fast. At school the next day, Sekai agrees to help Makoto by letting him "practice" on her, taking her to a karaoke and various other places, ending at the school roof. Makoto attempts to sleep with Sekai, and although Sekai protests at first, she soon consents. As Makoto starts to undress Sekai, he is interrupted by the arrival of a text message from Kotonoha. Later, Makoto and Sekai walk through the market, Makoto asks when they can continue practicing, but Sekai explains that it was enough practice and he should go use it on Kotonoha. Makoto bumps into Kotonoha along the way, and she asks him if he had received her message, but he declines. She apologizes and invites him to go to a water park in the near future. Makoto agrees, with Sekai looking on from afar with a sad look on her face.

The gang goes to the local pool for some fun. Hikari asks Makoto to give Taisuke a ticket because she is too shy to do it herself. At the pool, Makoto teaches Kotonoha how to swim; Taisuke is on the water slides with Hikari and Setsuna, while Sekai goes around swimming by herself. Makoto has completely fallen for Sekai, hinting her that he wants to be with her instead. Sekai keeps reminding him that he's Kotonoha's boyfriend and cannot be with her, and tells him to forget about the "practice". After everyone goes home, Makoto messages Sekai insisting that they meet. Despite her constant refusals, he travels to her home, finding her waiting in front of the train station for him. Makoto confesses his love for her and they have sex for the first time.

Sekai and Makoto plan to tell Kotonoha about what they have been doing in secret, but end up not doing so. Setsuna is worried about Sekai, knowing that she and Makoto are together, but Sekai doesn't look happy about it. Setsuna asks Sekai if Makoto has broken up with Kotonoha. Sekai lies, saying they have, but later Setsuna finds out that Sekai was lying. Otome plans to confess her love to Makoto, but Kotonoha, not knowing that Makoto doesn't love her anymore, tells Otome that she is Makoto's girlfriend. Otome text messages Nanami and asks if it is true. Later, when Makoto receives a text from Kotonoha saying that she will not be able to meet him at lunch, he and Sekai go to the roof. Kotonoha at the last second is able to go to the roof arriving just as Makoto begins undressing Sekai. When Sekai and Makoto confess their love to each other, Kotonoha overhears them from the door.

Kotonoha, refusing to believe that Makoto has cheated on her, still thinks that he loves her. Meanwhile, Tanaka, the other class rep besides Setsuna, has broken his arm, leading to Makoto replacing him. Otome and her friends guilt Kotonoha into getting more material for their class in order to build the hidden "lounge". Some girls from the basketball club have obtained a camera and the required equipment to film this "lounge". Setsuna, not knowing that Sekai was lying to her about Kotonoha and Makoto breaking up, tells everyone that they did, even taking Makoto's cellphone and blocking Kotonoha's calls and text messages. After hearing this, Taisuke tries to ask Kotonoha to go to the folk dance with him during the school festival while helping her carrying boxes, explaining that since she just broke up Makoto, they should go to the folk dance together. This conversation is overheard by Hikari, Nanami, and Makoto who happened to be in the hall nearby. Kotonoha then denies the break up and runs off, bumping into Makoto. Just when Kotonoha is going to talk to Makoto, Nanami steps in and blocks them, accusing Kotonoha of two-timing on Taisuke. Both Kotonoha and Hikari are left heart-broken and crying from the experience. Later during the night, Kotonoha goes to Makoto's house wanting to ask him to dance with her at the festival's bonfire, but sees Sekai's shoes by the door and leaves upset. Sekai feels guilty at having been caught and runs out of Makoto's house, bumping into Kotonoha and getting slapped by her.

Sekai stays home from school the next day, thinking about her encounter with Kotonoha the previous night. That day, Makoto goes and visits Sekai and tells her that what they are doing is not wrong, but Sekai keeps telling Makoto that cheating on Kotonoha is not the right thing to do. Soon after, Kotonoha dashes after Makoto at the train station, sitting by him in the train, and then asking him if he wants to touch her breasts, and if he would like to dance with her at the festival. On the way back to school with materials for the festival, Setsuna tells Makoto that she will be moving to France after the festival. After Makoto and Setsuna get back with the material, Makoto falls asleep and Setsuna kisses him. Kotonoha sees this while she passes by the classroom on her way home.

Setsuna and Kotonoha have a short conflict. Makoto flirts with Sekai the entire day, paying absolutely no attention to Kotonoha. When Makoto finally checks his cell phone, he finds twenty messages from Kotonoha who has been stuck minding her class' haunted house and "lounge," which is used by couples for sexual activities. On his way to finally see Kotonoha, he is stopped by Otome, who attempts to ask him to the bonfire dance later, only to be interrupted by Sekai. Back in the kitchen, Sekai asks if Makoto will dance with her at the bonfire. Otome encounters Taisuke, who is sulking over being rejected by Kotonoha. They discuss Makoto and Kotonoha's relationship status, and each resolving to be more assertive with their crushes. While Kotonoha is away from the lounge, Otome takes Makoto there, pushes herself onto him, and they have sex. Natsumi and Minami again bully Kotonoha about Makoto ignoring her, and she goes to his class room to see him. There she finds Taisuke who tells her that Makoto has already gone to the bonfire with Sekai. He then confesses to her and takes her into his arms. Makoto dances with Sekai around the bonfire, while Taisuke undresses and takes advantage of Kotonoha's fragile emotional state.

Kotonoha watches Makoto and Sekai dance and kiss in front of the bonfire and obviously upset, begins to cry. The next day on the train, Kotonoha sees Makoto but says nothing about Taisuke the evening before. Taisuke offers Kotonoha two tickets, with values that will be able to buy her lunches for several, but is rejected by her, again being told that she is Makoto's girlfriend. Taisuke is never seen again after this except for a brief scene at the end of episode 12. While she disposes of festival trash, she runs into Setsuna and Makoto. Setsuna forces him to break up with Kotonoha on the spot. Kotonoha threatens to reveal Setsuna's stolen kiss in the classroom, but Setsuna kisses Makoto. Makoto tells Kotonoha that he does not love her anymore, leaving her again weeping as Makoto abruptly leaves the area. Later, Setsuna goes to look for Makoto and overhears him in the gym store room, having sex with Otome. Kotonoha begins to act strangely at her home. Setsuna goes to Makoto's house and tries to get him to remain faithful to Sekai. Setsuna reveals that she hasn't told Sekai anything about finding him with Otome because she does not want to hurt Sekai's feelings. Makoto sees an opportunity and forces himself on her, knowing she has feelings for him. She tries to resist at first, and then consents as long as Makoto swears to break up with Otome. At the Girl's Basketball party, Sekai sees the video from the hidden camera in the "lounge" and sees the footage of Makoto with Otome. She tries to track down Makoto, and soon runs into Kotonoha on the bridge and is told that he is with Setsuna. Sekai notices that Kotonoha's eyes are completely lifeless. After the end credits, Setsuna is seen in the airport. Despite Makoto's attempts to find her, Setsuna ignores him and recalls the first day of school where Sekai, showing the first hint that she was in love with Makoto, successfully begged Setsuna to switch seats with her. With mixed feelings of sadness and regret, she says farewell.

Sekai is absent from school for several days, saddened by Setsuna's abrupt departure for France and Makoto's videotaped betrayal with Otome. It is revealed that Hikari has begun having sex with Makoto, who has not seen or attempted to contact Sekai since she started to be absent from school. Kotonoha is still in her lifeless, emotionally broken state, continuously phones and sends text messages to Makoto even though she is blocked. Natsumi, Kumi, and Minami want to know what it's like "having" Makoto, so the next day, they go to Makoto's house and they presumably have sex with him, though it was not shown. While scrolling through old text messages between herself, Setsuna, and Makoto, Sekai suddenly rushes to the bathroom and vomits. Concluding that she is pregnant, she goes to school to inform Makoto, loudly declaring for all in the class to hear that it is his child and insists he take responsibility. One by one, all the girls start to ignore Makoto, though Sekai warms to the fact that she is carrying his child. Kotonoha makes another call on her phone to Makoto, saying that she is waiting for him under the tree by the train station. Otome shows up at Makoto's apartement and tells him that he has changed from the Makoto she knew in Middle School, and not for the better. Afterwards, Makoto goes out for a walk and tries to call every girl in his speed dial, but finds all his calls have been blocked. He ends up where Kotonoha has been waiting for him, and upon seeing her current state, finally realizes the damage he's done to her. He apologizes to Kotonoha and hugs her; life returns to Kotonoha's eyes and she smiles warmly.

While having dinner with Kotonoha at a restaurant, Makoto receives a phone call from Sekai who’s waiting at his home. Before hanging up he tells Sekai that she ruined his life by getting pregnant and announcing it to the class. Sekai texts him an apology before heading home, but not before she destroys the dinner she had planned to share with Makoto. On the train ride home she catches sight of Kotonoha and Makoto together which causes her to head back to Makoto's. After a brief and tense confrontation with Kotonoha, Sekai leaves in a state of agony after witnessing Makoto and Kotonoha deep kiss in front of her. As she is walking home, it is revealed in a flashback that Sekai knew all along of Setsuna's feelings towards Makoto even to the point of supporting Setsuna to confess. It is only then that Sekai realizes what she has done to Setsuna. The situation worsens overnight when Makoto suggests to Sekai over text that she have an abortion. Overcome by emotion, Sekai stabs Makoto repeatively the next day while at his apartment. He briefly whispers Sekai's name, and promptly dies from the various stab wounds. After a few moments of staring at Makoto's body beneath her, Sekai becomes shocked by her actions and runs away in a panic. Shortly afterwards, Kotonoha arrives at Makoto's home to find him dead. Sekai, who was hiding in her room, receives a cryptic text from Makoto to meet "him" at the school's rooftop. She proceeds there cautiously, carrying the knife she killed Makoto within her jacket. After arriving she is confronted by Kotonoha, who accuses Sekai of lying about being pregnant. Sekai denies lying, and Kotonoha tells Sekai to look inside the gym bag lying on the bench, which contains Makoto's severed head. At that moment, Kotonoha unravels a bloody dozuki and kills Sekai. In a state of insanity she cuts open the womb of Sekai’s lifeless body and upon seeing nothing, concludes that Sekai was lying. On her family’s boat drifting off in the distant ocean, Kotonoha is last seen holding Makoto's head in her arms saying that now they can finally be together. Time passes by and life at the school returns to normal.