As children, Michiru and Shuta, who carried Saku because he has a weak heart, climbed to the top of a hill and, saw a rainbow there together.

Years later Michiru decides to go to England to study but she can't bring herself to tell them face-to-face.

Shuta takes a train home while wearing the watch and is surprised to find he has returned in time to December 20, 2010, the day before Saku's death.

To please Michiru, Shuta agrees to put up prisms in the lab's windows to create a rainbow effect.

Matsudo, his eccentric former science teacher, and discovers that the future has been altered slightly, because Matsudo now remembers the prisms in the window.. On the third time he goes back in time, Shuta goes again to the school and sees a shamanistic ritual through a window that is supposed to summon Kyoko, the spirit of a girl who committed suicide there.

He repeats the routine for that 2012 morning, then, after traveling back 2 years as usual, runs to the school and claims he doesn't know how he disappeared from Saku's and Michiru's presence.

Kyoko warns Shuta time is a law of nature and messing with it could make the future worse rather than better.

He find that Saku's mother has taken an overdose of pills and collapsed at her home and realizes what Kyoko's had meant.