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.

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.

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.

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.

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.

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.