In the city of Fujisawa, Kaito Ishikawa portrays Sakuta Azusagawa, a high school student navigating a relatively normal life that is heartfelt and filled with meaningful relationships. As a second-year student, Sakuta finds his days made brighter by the company of Asami Seto, who voices Mai Sakurajima, his girlfriend and senior at school. Mai is well-loved by those around her, yet she, too, enjoys the quiet moments they share together, creating a peaceful bubble amid the chaos of adolescence. Their relationship is a core element of his daily routine, and they cherish their time together, unaware of the upheaval that is looming.
However, the calm of their lives is unexpectedly shattered when Sakuta encounters a figure from his past — his first love, Shoko Makinohara. Shoko's character, subtly played by Inori Minase, appears in a complex double — with an older version of herself and a middle school version, each with distinct and intertwined stories. Sakuta's discovery about these two Shokos marks the beginning of a deeply emotional and intricate journey. The middle school Shoko is battling a life-threatening heart disease that urgently requires a transplant. Her condition is so critical that she is in immediate danger, and the situation becomes even more complicated when Sakuta learns that the adult Shoko came from the future after receiving a successful heart transplant. The donor for her transplant was none other than Sakuta himself, who had been declared brain dead following a tragic traffic accident on Christmas Eve.
As Sakuta and Mai become deeply concerned about Shoko's fate, Sakuta begins uncovering the connection between his wounds and the adult Shoko. It turns out that her feelings toward him grew in her youth because of the circumstances surrounding her recovery. Driven by an intense desire to save her, adult Shoko confesses that she came back to the past out of love, and that her feelings are rooted in the bond they formed through her struggles. This revelation pushes Sakuta to make a heartbreaking decision: he chooses to sacrifice himself to give Shoko a chance to live.
In a daring attempt to alter fate, Sakuta nearly gets hit by a car, but at the last moment, Mai pushes him aside, taking the impact herself. Her sacrifice has profound consequences — while Sakuta is spared, Mai tragically dies, becoming Shoko's heart donor. Her death strikes Sakuta deeply, leaving him devastated and overwhelmed by grief. Adult Shoko, witnessing his suffering, reveals a crucial secret: she can help him go back in time to rewrite history and prevent the tragedies. However, this comes with a heartbreaking cost — her own existence in the present will vanish if the past is changed.
Faced with impossible choices, Sakuta resolves to go back to the past once more to save his love. His journey takes him to Christmas Eve, echoing the moment when the tragedy initially struck. In an emotional, desperate effort, he wears a pink rabbit costume inspired by Mai's own playful spirit and heads to the train station, searching for a way to influence events. During this time, characters such as Yuma Uchida, Satomi Sato, and others become part of the intricate web of interactions. When Koga questions Sakuta about his strange outfit, he realizes that Mai, Koga, Nodoka, and Shoko can all see him, which sparks hope. Using his phone, Sakuta contacts his past self to warn him and sets out to change the course of events.
He manages to arrive at the crucial moment, pushing his past self away from the incoming car to prevent the accident. The act succeeds in saving both Mai and Sakuta's younger self, but it also creates a paradox: Mai's death is avoided, but without her sacrifice, Shoko's chance for a transplant disappears, dooming her to die. Sakuta refuses to accept this outcome. Together with Mai, he vows to find another way. They are determined to help young Shoko survive, even if that means risking everything they have fought for throughout the story.
As the story reaches its poignant climax, Sakuta and Mai revisit the scene in a bid to rewrite the future. In a touching moment, young Shoko reveals that she has come to understand everything that has happened. She chooses to create a future where she never meets Sakuta or Mai, believing that this will spare everyone from pain — a heartbreaking decision that ultimately leads to her passing away early in life.
Time seems to rewind once more, returning to a time when Shoko was in fourth grade. She bravely writes her future hopes, choosing to forget her past meetings with Sakuta and Mai, resigning herself to a lonely but peaceful existence. Yet, as the story concludes, it is told that in the present day, Mai, who starred in a film based on Shoko's story, has forgotten her connection to Shoko. Still, on New Year’s Day, Sakuta and Mai visit a shrine, passing by a beach where, for the first time, they encounter a healthy, young Shoko. Through fragments of memories crossing different timelines, Sakuta and Shoko gradually recognize each other, hinting at the enduring possibility of connection and the hope that love finds a way beyond even the greatest of time’s barriers.
