Kaori and Sou have been friends since childhood, so naturally everyone assumes they're destined to be together. But after a tragic accident cuts Kaori's life short, these star-crossed members of the Astronomy Club become entangled in a web of overlapping timelines and parallel universes.

When the clock resets after Kaori's death, her week begins just as it did before�only this time, as Sou and the other members of the Astronomy Club investigate a few mysterious rumors floating around the school, they stumble upon a very unconscious and very naked girl named Yui. She claims she has no memory of where she came from, but in reality she's arrived from the future with an important mission: change Kaori's fate and ensure a brighter future for Sou and the rest the Astronomy Club.