Before he leaves the town of Sakuranomori, Sana Hidaka has a party with his friends, Syusuke Wakatsuki, Nanaka Yatsuhiro, Aoi Oribe, and Syuri Watatsuki. They each give him going away presents, with Nanaka playing a half-completed song on her violin. Five years later, Sana returns to Sakuranomori, staying at an apartment owned by Aoi's parents. When he goes to school the next day, he recognizes two of his childhood friends: Syusuke and Syuri. But when Nanaka enters the classroom, Sana fails to recognize her. As a result, he is furiously slapped by Nanaka.

Sana mulls over Nanaka's reaction, and is elected the classroom representative along with Nanaka when Syusuke refused to be the classroom representative for another year. Nanaka still acts coldly towards Sana, leaving him lost in the library. When he returns to the classroom with the aid of Asami Hoshino, she looks jealously at him and Asami. Later, Sana and Nanaka coincidentally go to the music room, where Nanaka, sequestered in the teacher's office, overhears Sana playing the song she played to him on her violin the day he left. Touched, she says hello to him the following morning.

In order to repair Sana's and Nanaka's relationship, Syusuke and Syuri decide to go on a trip to a river that the five used to go to in their youth. Beforehand, Sana accidentally breaks Nanaka's mirror, and Nanaka is appreciative when Sana buys her a new one, but forlorn after learning that Sana bought one for Asami as well. After the group goes to the river, they swim at the spa, where Nanaka is embarrassed at her lackluster swimsuit. As Nanaka is going home with Sana and Aoi, she reveals she made cookies for the group, and the three eat them at Aoi's apartment. There, Sana gives Nanaka a pink stone he found near the river as a gift.

On his way back from school, Syusuke comes upon a group of boys tormenting a young elementary school girl. Syusuke chases the boys off, and the next day, the girl, Hinako Mochida, thanks Syusuke. Shortly afterwards, Hinako meets Sana at a restaurant and reveals her infatuation with Syusuke. With Aoi's aid, Hinako attempts to court Syusuke, but despite her efforts, is rejected by him. Despite this, Syusuke reveals that he considered Hinako an adult, which Hinako tearfully accounts to Sana, noting her satisfaction nevertheless.

Nanaka begins to spend increasing amounts of time practicing for the upcoming festival at the temple, where she will perform a traditional dance so the sakura will bloom that year. However, she forsakes her practice to go with Sana, Syuri, and Asami to help at a retirement home. After meeting a woman traumatized by the death of her granddaughter, Nanaka mysteriously refuses to play her violin to cheer her up. Later at the festival, the group observes Nanaka perform her dance, and Sana sees her afterwards, expressing his delight at her dancing.

After learning that her mother will not be home for her birthday, Hinako moves into Sana's apartment. Hinako's inexpertness in cooking and Sana's shock at her arrival contributes to the hectic situation, but Sana manages to get her comfortably situated into the room. She was not able to sleep without her Yeti giant soft toy and asks Sana to be her Yeti and let her hug him. Sana eventually learns that the day was Hinako's birthday, and the following day, takes her around town with Syusuke. Once they return to Sana's apartment, Nanaka, Aoi, and Syuri have prepared a birthday party for Hinako. After Hinako leaves the party, she thanks Sana for the only birthday party she has ever had with a kiss on the cheek before running home.

Due to Sana and Nanaka's work as classroom representatives, their sensei, Yuzuki Fujimura frequently gives them caramels, causing Sana to remember an incident in a youth when a younger Yuzuki gave him a caramel. Meanwhile, Syuri attempts to gather signatures for a project to stop the building of a new town hall over the nature center. The school's administration forbids Syuri to continue. At her home, her father, the one who proposed the construction, angrily berates Syuri, who flees home. At the retirement home, Sana and Asami continue to help there, and fulfill a request by an elder lady to bury a package on a hill. However, Asami trips and Nanaka comes upon Sana and Asami accidentally embracing each other.

Nanaka acts coldly towards Sana the following day, even after Sana explains his incident with Asami was an accident. After Sana exclaims that Nanaka is not her old self, she angrily runs away. Sana comes upon an old mailbox that Sana and Nanaka used for private communication in their youth, and finds dozens of letters, some desperate cries for help. Sana then learns from Syusuke that three years prior, Nanaka's house had burned down, killing both of her parents and traumatizing her. After learning of Yuzuki's ability to cheer herself up with caramel, Sana meets Nanaka, who tearfully denounces him for not being there for her when she needed him most.

As Nanaka contemplates how to apologize to Sana, she goes into shock after accidentally burning the fish she was cooking, bringing her close to recollecting memories of the fire that consumed her house. She then overhears a discussion between her uncle and her aunt hinting that her uncle was the one that caused the fire. The next day, the group goes to a theme park, where they meet Asami and her cousin. After viewing a tokusatsu show, Nanaka gathers enough courage to talk to Sana. As they ascend in a ferris wheel, the two reconcile. In another car, Syuri and Syusuke angrily argue over their stepmother, and Asami accidentally catches a glimpse of Syuri's saddened face.

Syuri and Syusuke travel to the grave of their mother to pay their respects, each buying each other presents afterwards to cheer themselves up. When they return to school the next day, an anonymous letter claims that Syuri and Syusuke spent the night in a Love Hotel. Although the school refuses to pursue this, Syuri and Syusuke's father angrily beats Syusuke for this. Syuri, Asami, and Sana then go to give a gift to one of the ladies from the retirement home. However, the lady attempts to kill Syuri, and is stopped by Asami, who is stabbed in the process. As a result, Syuri's father plans to send Syuri to London to save his political career, and Syusuke promises to join her so they can be together.

Sana visits Asami in the hospital, where Asami reveals that she was the one who sent the anonymous letter claiming that Syuri and Syusuke were in a love hotel. She admits that ever since Syuri rejected her for expressing her feelings towards Syuri, Asami has constantly hated her. When Syuri and Syusuke arrive to visit Asami, she acts normally, although she asks Syuri to converse privately with her. Syuri later runs out of the hospital in tears. The following day, Syuri and Syusuke are absent from school, and Sana realizes that they are running away to avoid being separated. He desperately runs after their train, and reminiscent of Syusuke when Sana left town, Sana yells that they will always be friends, and Syusuke and Syuri erupt in tears.

Sana, Nanaka, and Aoi are depressed due to Syuri and Syusuke leaving, and decide to go to the river to take their minds away from the subject. However, Aoi fakes being ill to allow Sana and Nanaka to be together. After a failed attempt at fishing, Sana takes Nanaka to a cave that he and Syusuke considered their secret hideaway. After it starts to rain outside, the two stay in the cave, where Sana asks Nanaka why she did not send the notes in the mailbox directly to Sana. Nanaka reveals that she was afraid that the Sana she knew and loved was gone. The following morning, as Nanaka walks outside, she recalls what occurred in the fire in her youth and screams in pain.

Nanaka recalls that on the night of the fire, her father had been told that Nanaka was not his child, and in turn, he beat his wife and set the house on fire. In response, Nanaka locks herself in her room and refuses visitors, and Sana visits every day in an attempt to talk to her. One day, Sana finds Nanaka in the bath with her wrist cut, and he manages to have her brought to a hospital. In a subsequent conversation, Sana reveals that he attempted suicide also in his old middle school, and came back to Sakuranomori in order to be with his friends. Ten years later, everyone returns to see a concert performed by Nanaka. Before the concert, Sana proposes to Nanaka, who accepts. For the concert, Nanaka plays the finished version of the song she wrote for Sana when she was younger titled "Myself ; Yourself". Shusuke and Shuri attend the concert though unexplained on their return.