Sou was a mercenary who piloted a Simulacrum—a combat machine—until an incident took away his ability to fight and his employment along with it. He goes home to his foster father's island, Wadatsumi, looking for a quieter life. His former partner Yuuri follows without explanation. Shortly after arriving, he rescues a girl with no memory—Tsukuyomi—who ends up staying at the house. He accepts a job offer from an anonymous client that requires him to go undercover at the island's only school, Kouka Academy, and investigate whether its students are connected to the series of terrorist incidents that have recently hit the island. The student council president Mao and her Simulacrum block his way in. Then a self-described fairy appears and merges with his machine, changing what he is capable of.