In Fujisawa, Sakuta Azusagawa is in his second year of high school. He greatly enjoys his days with Mai Sakurajima, who for the past 6 months has been both his girlfriend and senior. However, their peaceful lives are interrupted by the appearance of Sakuta's first love, Shoko Makinohara. While taking his younger sister to the hospital, Sakuta discovers that there are two Shokos — the one in middle school and the older one who helped him in the past. Sakuta discovers that the middle school Shoko is suffering from a heart disease that urgently requires heart transplantation to save her life. When both Sakuta and Mai become concerned about the middle school Shoko's situation and think of ways to help her, Sakuta eventually figures out the link between his wounds and adult Shoko. Adult Shoko comes from the future after a successful heart transplant, and her heart donor was Sakuta himself — who Shoko reveals was declared brain dead after a near-fatal traffic accident on Christmas Eve. However, Shoko developed feelings for Sakuta through their interactions in her youth, so she decided to come back to the past and save his life. Against Mai’s wishes, Sakuta decides to sacrifice himself so that Shoko can live. Sakuta nearly gets run over, but Mai pushes him aside at the last moment and gets hit by the car instead. As a result, the entire chain of causality changes — Sakuta lives, but Mai is killed and becomes Shoko's heart donor instead.
