Hibiki Otonokoji lives on the island of Utsuroshima with her best friend, Yuri Kagarin, and 14 other people. The islanders have decided to separated themselves into two groups: the ones who CAN fight off the creatures that have randomly appeared by the shore, and the ones who cannot.
The "Rumblers," as the ones who can't fight call them, mostly look down on the "Tumblers," and vice versa. The one exception from the Rumblers is Yuri, who is surprisingly a good fighter; and the one exception from the Tumblers is poor Hibiki, who is trying to get stronger to see her friend again
The two are forced to separate, until tragic circumstances bring them together, and away from the island...and possibly, even away from the reality they knew.