High school student Sakuta Azusagawa lives in the town of Fujisawa alongside his girlfriend, Mai Sakurajima, an older student and former child actress who has returned to public life. Their relatively peaceful days are interrupted by the reappearance of Shoko Makinohara, a mysterious figure who was Sakuta’s first love in middle school and once helped him through a difficult period. Sakuta soon discovers there are two versions of Shoko: a cheerful middle school girl who is suffering from a grave heart condition that requires a transplant, and an older woman from the future.

The phenomenon known as Adolescence Syndrome, which has previously manifested in Sakuta’s friends as time loops and body swaps, now presents itself as a complex temporal paradox linked to the two Shokos. Sakuta learns that in a future timeline, he died in a traffic accident on Christmas Eve, and his heart was donated to save the younger Shoko’s life. The older Shoko, having survived with his heart, traveled back in time to try and prevent his death. When Sakuta attempts to sacrifice himself to save the young girl, Mai pushes him out of the way at the last moment, dying in his place and becoming the heart donor instead.

Devastated by Mai’s death, Sakuta works with the older Shoko to alter the past once more. He returns to Christmas Eve and, with the help of a friend who can perceive him, successfully prevents the accident from claiming either his life or Mai’s. This outcome, however, means no heart is available for the younger Shoko, who is now destined to die. Unwilling to accept this, Sakuta and Mai vow to find another way to save her, even if it means risking their own future together. The younger Shoko, aware of everything that has transpired, chooses to erase the timeline where she ever met Sakuta and Mai, wishing for them to live without the sadness her illness has caused.

In the end, time resets to the present, and Sakuta and Mai’s lives continue normally. While their memories of Shoko have faded, they encounter a healthy young girl on the beach, and without any clear reason, Sakuta recognizes her. The narrative explores the boundaries of sacrifice, the desire to protect those one loves, and the lingering power of connections that persist even across rewritten timelines.