Middle school students Futaba Yoshioka (Natsuki Deguchi) and Kou Mabuchi (Kaede Sakurai) find themselves drawn to each other after a rainy day. Whenever their eyes meet, they quickly look away, only to meet again. However, Kou suddenly transfers without saying anything. As they become high school students, Futaba, trying not to be disliked by her friends, starts to notice Makita Yuriko (Riko), who is isolated in class. Before Futaba, a boy similar to Kou appears, but he is called "Mabuchi".

Teasing Futaba, who is at a loss for an answer, Kou says he isn't "the Tanaka-kun from back then." Doubts grow in Futaba about what happened during the three years they couldn't meet. Meanwhile, Futaba and others are assigned pool cleaning as part of their training task. Shuko ends up alone with the teacher she harbors feelings for, Yoichi Tanaka (Daiki Kenkon), but is urged to return to the group. Futaba worries about Shuko's return, but is pushed away as annoying, and then Yuriko gets injured.

Futaba realizes that she knows "the current Kou" rather than "the Tanaka-kun from back then." As she begins to notice her feelings for Kou, she also learns that Yuriko likes Kou. Futaba agonizes over this but decides that she doesn't want to sever her feelings for Kou and resolves to confront Yuriko directly. Meanwhile, Futaba witnesses Kou staying out late at night with his local friends, including Yudai Kodama (Keisuke Watanabe), and worries about him. Frustrated with Kou's continued reluctance to open up, Futaba takes a certain action—.

Futaba and the others plan a study session for Kou, but his heart remains closed off. During this time, Futaba learns that Kou's mother, Akari (Keiko Horiuchi), has passed away. Confronted with Kou's expression, which seems to have given up on seeking and expecting anything, Futaba decides to learn about Kou's past from Yoichi. Wanting to truly understand and support Kou from the bottom of her heart, Futaba resolves to open the door to Kou's heart.

With Kou's confession as a catalyst, Futaba and Kou steadily shorten the distance between their hearts. Encouraged by Futaba, Kou also opens up honestly to Yoichi. Amidst this, Futaba and the others decide to go to the summer festival together. On the day of the festival, Yuriko appears in a yukata and actively approaches Kou. On the other hand, Toma Kikuchi (Ryosuke Sota), who came to the festival with his friends, reunites with Futaba and starts to be drawn to her positive attitude. As their feelings intertwine, Yuriko confesses her feelings to Kou.

Once again, they promised to meet at the Festival. Futaba and Kou, who had come within a millimeter of fulfilling their promise, but it never came to fruition. As summer vacation ends, Futaba confronts Kou, who had been unreachable, but his expression is different from before. Meanwhile, as preparations steadily progress for the cultural festival, Toma, who has begun to harbor feelings for Futaba, actively closes the distance despite knowing Futaba's feelings for Kou. After school, Futaba and Toma find themselves alone in an empty classroom.

Futaba, who was embraced by Kou, is interrupted by a call from a girl who calls Kou "Kou-chan." As the cultural festival approaches, Futaba, invited to Toma's live performance, finds herself moved to tears by Toma's song, which resonates with her current feelings. Kou, who is with her, tries to speak to Futaba, but... On the other hand, Shuko, who has come to Yoichi's place, takes an unexpected action. Just one millimeter away from youth—will their feelings for each other truly come to fruition?