The series follows Sousuke Sagara, a member of a covert anti-terrorist private military organization known as Mithril, tasked with protecting Kaname Chidori, a spirited Japanese high school girl. He moves to Japan to study at Chidori's school, Jindai High School, with assistance from his comrades Kurz Weber and Melissa Mao. Having never experienced social interactions, Sousuke is seen as a military maniac by his schoolmates as he interprets everyday situations from a combat perspective. He comes to relate with Chidori who realizes that Sousuke is protecting her, but he does not reveal the reasons due to orders as well as the fact that he does not know why Chidori is being targeted by different organizations.