Four close friends — the kind-hearted Natsuki Hashiba, the charming Tomoya Matsunaga, the enthusiastic otaku Tsuyoshi Naoe, and the rather sadistic Keiichi Katakura — embark on their second year of high school filled with adventures and challenges. As spring unfolds, Natsuki gathers the courage to approach Anna Kobayakawa, who graciously offers him her hand towel after he falls into a puddle. However, his attempt to return the towel is thwarted by Mari Tsutsui, Anna's protective best friend. Consequently, the boys wager that Natsuki must ask Anna out by the end of the semester. To everyone's surprise, he manages to get her Line ID, much to the dismay of Mari.
As summer arrives, Natsuki, Tomoya, and Keiichi face the harsh reality of failing their exams. They seek assistance from Tsuyoshi and his girlfriend Yukiko Asai, inviting Anna to join them. Eventually, Yukiko finds out that Tsuyoshi has ambitions of applying to a university in Tokyo, adding another layer of complexity to the boys' summer. After passing their remedial exams, Natsuki, Tomoya, and Keiichi accompany the girls to a summer festival. During this time, Natsuki grows closer to Anna, while Tomoya realizes that Mari fears he might claim Anna for himself.
As autumn sets in, tensions rise during guidance counseling when Tsuyoshi feels torn between his university aspirations and maintaining his relationship with Yukiko. Anna becomes perplexed when she witnesses Natsuki's classmate, Reiko Chiba, seemingly getting too close to him. She's later rescued from unwanted advances by Keiichi, who pretends to be her boyfriend in a protective gesture.
When winter comes, Tomoya skillfully distracts Mari to allow Natsuki and Anna some time together. He then extends an invitation for her to join a Christmas party, which unexpectedly turns out to be a surprise birthday celebration for Mari, orchestrated by Natsuki and Anna. Finding this revelation upsetting, Mari leaves in haste, prompting a search among their friends. During the search, Tomoya finds her and, in a revealing moment, she confesses her fear of losing her closest friend, Anna, leading to an unexpected kiss. Meanwhile, Natsuki develops a fever during the search and unintentionally kisses Anna.
Post winter break, the dynamics between Natsuki and Anna become noticeably awkward. Meanwhile, Tomoya confesses to Mari that he has developed feelings for her. However, after an incident knocks her unconscious, Tomoya carries her to the nurse’s office. He assures her older brother, Masaomi Tsutsui, of his earnest intentions, leading Masaomi to relay this to Mari. Realizing the depth of Tomoya's feelings after he deletes contact information for other girls, Mari begins to reconsider her own feelings.
As spring arrives and the boys prepare to enter their third year, Anna mistakenly overhears Natsuki’s supportive words to Chiba regarding a rehearsal for their play. Misinterpreting the conversation as a love confession, she begins to distance herself from him. In a twist of fate, Keiichi declares that he will pursue Anna if Natsuki does not act soon, igniting a heated argument between Tomoya and him. Upon receiving a timely warning from Tsuyoshi about Keiichi’s looming confession, Natsuki chases after them. Keiichi's threat turns out to be a ruse as he grants Natsuki and Anna some private time within a planetarium exhibit. It is there that they finally confess their love for each other and share a heartfelt kiss. Outside, the rain dissipates, revealing a breathtaking rainbow in the sky as a symbol of their blooming relationship.
In a delightful mid-credits scene, the boys joyfully leap into the school swimming pool, celebrating their friendship as they approach graduation from high school, marking the end of one chapter and the start of another in their lives.
