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".

Encouraged by Kou, Futaba decides to start over her high school life in a new class. Determined, when Futaba volunteers as class representative, Kou also steps up to become another class representative. Then, Shuko Muraoka (Ayara Shida), Aya Kominato (Yasutaka Niibata), and Yuriko join Futaba as event committee members to participate in a training session. However, the opinions of the five never align, leaving Futaba cornered during the presentation. Who extends a helping hand to Futaba in her time of need—?

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?

Futaba Yoshioka (Natsuki Deguchi) and Kou Mabuchi (Kaede Sakurai) gradually draw closer to each other's feelings and come into close contact on the day of the cultural festival. Futaba, who wants to know Kou's true feelings, resolves to convey her feelings to him. Just then, Yui Narumi (Karin Fujiyoshi), a classmate Kou met in Nagasaki when he transferred during middle school and who shares the same sadness, appears before them. Unable to leave Yui, who knows the same sadness, alone, Kou embraces her in front of Futaba and Toma Kikuchi (Ryosuke Sota). Their youth is just one millimeter away, yet so distant.

Futaba, worried that Kou's progress might halt again due to Yui's presence, who knows about Kou's past, confronts Yui with her feelings, fearing that they might not be for Kou's sake but for her own. Amidst this, a "suspicion" arises about Yoichi Tanaka (Daiki Kenkon). Shuko Muraoka (Ayara Shida), harboring steadfast feelings, calls Yoichi to the rooftop to confess her feelings. And then, Futaba also opens up her feelings to Kou

Futaba's heart sways greatly at Toma's straightforward words, and the distance between them rapidly closes. Meanwhile, as Kou recalls painful memories with his mother, Akari (Keiko Horiuchi), he once again realizes his feelings for Futaba. Encouraged by Aya Kominato (Yasutaka Niibata), Kou decides to face his honest feelings and meet Yui again to sever any lingering emotions of sympathy. In the midst of this, Toma presses Futaba for an answer to his confession. What answer will Futaba come up with?

Futaba has started dating Toma. Meanwhile, the time for the school trip is approaching. Coincidentally, the destination is Nagasaki, where Kou spent his final days with Akari. Witnessing the intimate relationship between Futaba and Toma, Kou feels uncomfortable. On the other hand, developments begin to unfold in the relationships between Shuko and Kominato, Makita Yuriko (Riko) and Naigu Haruhiko (Miyazato Soru). Kou decides to take action to create new memories in Nagasaki.

Despite feeling bewildered by Kou's invitation, Futaba decides to accompany him as a representative of their friends. Kou tries to move forward to overcome his sadness as they walk through the streets of Nagasaki. Futaba's heart wavers as she tries to support Kou, but she begins to feel guilty towards Toma. Later, Futaba honestly tells Toma about sneaking out with Kou during the school trip and promises to face their issues together. On the other hand, Kou, in an attempt to establish boundaries with Yui, meets with her—.

Unable to erase Kou from her heart, Futaba heads to Toma to bid farewell. However, Kou, without knowing about Futaba's decision, stops himself from reaching out to her. Futaba and Kou continue to pass by each other... And then comes the Christmas night. Amidst the intertwining of their feelings, Futaba visits the place filled with memories of Kou. Over and over again, it felt like it would start but never did, between Futaba and Kou. Near yet far, will their youth, just one millimeter away, truly reach its conclusion?