Vesper is a covert organization fighting a war against alien invaders using purpose-built battle androids. Mahoro is their most effective unit — she has won more engagements than any other — but her operating time is almost gone. Rather than burn through what little remains in continued combat, she is offered a choice: live out the time she has in peace. Mahoro chooses to spend it as a maid for Suguru, a middle school student living alone after the death of his family, whose home has reached a state of disorder that only someone with her particular capabilities can address.