The narrative begins during the bustling Christmas season in Fujisawa, where high schooler Sakuta Azusagawa enjoys a peaceful relationship with his girlfriend, Mai Sakurajima. Their tranquility is suddenly disrupted when Sakuta encounters two monumental dilemmas rooted in the supernatural phenomenon known as "Puberty Syndrome." First, Sakuta's
elementary school tutor and first crush, Shoko Makinohara, mysteriously reappears in two distinct forms: a healthy middle-schooler and a terminally ill adult woman in urgent need of a heart transplant. Meanwhile, a parallel timeline reveals that a young boy died in a tragic
traffic accident, and his heart was miraculously donated to adult Shoko, allowing her to survive. This donor is revealed to be Sakuta himself, who dies in that same accident. Adult Shoko has traveled back in time using her own mysterious Puberty Syndrome to prevent Sakuta’s death, though doing so means sacrificing her younger self's existence and changing the timeline so that Sakuta never meets her, nor does he develop the deep bond that defines his current life.As the emotional weight of these revelations sets in, tragedy strikes closer to home. While walking home together, Mai pushes Sakuta out of the path of a speeding car, absorbing the fatal impact herself. Stricken with agonizing grief, Sakuta spirals into despair until he is approached by middle-school Shoko. She offers him a desperate chance at salvation, utilizing her own timeline-altering abilities to send Sakuta back to the fateful day of the accident. Armed with foresight and driven by his fierce love for Mai, Sakuta races against time. He successfully intercepts Mai before the accident can occur, altering the sequence of events and ensuring her survival. However, by changing the past to save Mai, adult Shoko begins to fade from existence, and with her, everyone’s memories of her are erased.Months later, on the beach during a brisk New Year’s visit, Sakuta experiences a powerful emotional resonance, haunted by a lingering sense of loss for someone he cannot quite remember. As the tide rolls in, he crosses paths with a healthy young girl who looks remarkably familiar. This chance encounter triggers a flood of emotional realization, bridging the gap between lost timelines and healing old wounds. Sakuta, Mai, and Shoko are finally reunited under a clear sky, ready to embrace a future they fought so hard to secure together, forever changed by the sacrifices of the past.