To combat the rising crime rate, Japan establishes Tokyo Butei High, an exclusive institution where "Butei," or armed detectives, perfect their lethal talents in the hopes of becoming mercenary-like agents of justice. Kinji Tooyama, an anti-social and brusque sophomore dropout who was previously a student of the combat-centric Assault Division, is one such Butei. Kinji now spends his free time studying logistics in order to conceal his powerful but humiliating special skill. However, his quiet days are cut short when he becomes the target of the famed "Butei Killer," and encounters an emotional storm and outspoken genius of the highest rank, Aria Holmes Kanzaki, who saves Kinji's life and insists that he become her partner after witnessing what he is capable of.