Sousuke Sagara transfers into Jindai High School posing as an ordinary student, but he's really an elite Mithril soldier assigned to secretly protect classmate Kaname Chidori from kidnappers who want the knowledge locked inside her as one of the mysterious Whispered. Raised from childhood as a combatant, Sousuke has no idea how to act like a normal teenager, and his overcorrections, from searching desks for explosives to treating gym class like a hostile op, spiral into constant chaos around Kaname. Outside school, Mithril's submarine crew and their Arm Slave mechs stand ready when the threats Sousuke is guarding against turn openly violent.