Fourteen-year-old Sheena Totsuki has been surrounded by war and death for as long as she remembers. She is forced to train with fellow orphans as child soldiers to fight tooth and nail in a conflict that has dragged on for too long. Casualties are a common occurrence, as is the case for Sheena's roommate one tragic day.

That night, Sheena encounters a strange girl covered in blood with a contradictory joyous demeanor. The next day, the strange girl is introduced to Sheena's class as Mimi Kagari, and rumors start floating around that she could be the orphanage's infamous secret weapon of the same name. Appointed as Sheena's new roommate, Mimi immediately grows attached to her, while Sheena tries to sort out her own feelings on the matter. Mimi seems mysteriously unfamiliar with the school and everyday life as a student, so Sheena finds herself becoming something of a caretaker.

Despite barren circumstances, the disparate pair grows closer. However, Sheena's worldview begins to crumble after she learns the truth behind Mimi's unimaginable power.

[Written by MAL Rewrite]