Shuta Jogasaki has been friends with Saku Kijima and Michiru Ando since childhood. As youngsters, the trio shared a tender moment on a hilltop where Michiru, Shuta, and Saku climbed together to catch sight of a rainbow, an image that becomes a quiet symbol of their bond. Years later, Michiru Ando makes the difficult choice to study in England. She keeps her plans to herself, delivering the news only to Shuta Jogasaki in a sealed letter that asks him to hand it to Saku Kijima on the day she leaves, December 21, 2010.
On the road to delivering that letter, fate intervenes: Shuta’s bicycle breaks down, and he pleads to borrow Saku Kijima’s bike to reach a basketball game. In a cruel twist of destiny, Saku Kijima dies while sprinting toward the station to see Michiru off. The weight of that loss settles on Shuta, who feels a stubborn guilt for what happened. After Saku’s death, Shuta and Michiru drift apart, their contact fading into the background as time moves on.
Two years pass. On December 20, 2012, Shuta attends Saku’s memorial and is drawn into a strange moment when he wears a mysterious watch—named in the lore as a “You're a Time Traveler” device—that seems to rewind events. He finds himself back at the morning of December 20, 2010, as if the clock had been turned. A transit to the past becomes his new reality as he makes his way to a school where a professor, Professor Matsudo, now remembers the prisms in a lab window that create a rainbow effect. The strange chain of events intensifies when a basketball team member, angered that Shuta has not joined practice, hurls a ball at him and he suddenly slips back to the present.
The time travel intensifies on a third loop: Shuta is drawn to a shamanistic ritual unfolding behind a classroom door, a doorway through which the spirit of Kyoko appears. This encounter disturbs the timeline and sends him racing back to the morning of December 20, 2012. In this cycle, the 2012 version of Shuta questions the integrity of the past as he races toward a future that now feels fragile. A hypnotic session with the science teacher digs up a hidden truth: Kyoko is not a mere legend but a “time prisoner”—visible only to willing time-travelers. She warns that time is a natural law, and meddling with it could erase more than memories. Yet she agrees to teach him the cautious path, known as the “Jack Fin” technique: change as little as possible and avoid dramatic shifts.
That night, the purpose of Michiru’s departure weighs on Shuta. He attempts to coax Michiru into confessing her England plans, but she resists, maintaining her resolve. When midnight arrives, Shuta discovers a devastating consequence: as December 20 turns into December 21, 2010, the dimension he travels back from becomes a memory he cannot fully hold onto. Saku’s mother’s heartbreak becomes a clue to the severity of interference, and Kyoko pleads with him never to meddle again; if he does, everyone’s memories—including his own—could vanish. Yet Shuta makes a heart-wrenching choice to sacrifice himself in order to save Saku, and Kyoko passes the watch on to him.
In the next arrival, the Shuta who enters the basketball court is the 2010 version, unarmed with the watch and untouched by the jolting events that would follow. The 2012 Shuta confirms that Michiru’s letter truly reveals her feelings, and he races toward the station with Saku on a bike, determined to reconnect with Michiru. He reads the letter and learns that Michiru loves him, not Saku, and that he was the one who could have saved Saku’s life all along. After this act of courage, the memories of Shuta fade from everyone’s minds—except for Kyoko, who remains the one constant link. She cannot leave the school, yet she vows never to forget him.
Years later, on a sunlit beach, Shuta is seen again, quietly holding a quarts prism. Saku and Michiru appear in the distance, walking hand in hand, while Michiru notices the glimmer of the prism and thanks him for the small, shared memory that has endured. The moment crystallizes into a bittersweet ending: Shuta gifts the prism to Michiru, a symbol of their brief, timeless connection, and they part ways as strangers again, with only Kyoko’s unfading memory of him keeping the thread of their story alive.
