When the rest of the residents on her home island died in one night five years ago, young Mari survived but had her memory erased. Now enrolled in the exclusive Kaihou Academy, Mari meets Hagino, a girl who holds the key to her past. Unbeknownst to Mari, Hagino belongs to the alien group who attacked her island and now passes for human. As the two become friends, Hagino questions her mission to facilitate another invasion in this anime series.