As a reaction to the rising crime rate, Japan establishes Tokyo Butei High, an elite academy where "Butei"—armed detectives—sharpen their lethal skills with the aim of becoming mercenary-style agents of justice. Among them is Kinji Tooyama, a withdrawn and blunt sophomore dropout who formerly attended the combat-focused Assault Division. Kinji now leads a leisurely life, studying logistics to conceal his potent yet embarrassing special ability. Yet his tranquil existence is abruptly disrupted when he becomes the target of the notorious "Butei Killer" and crosses paths with Aria Holmes Kanzaki, an emotional whirlwind and outspoken prodigy of the highest rank. Aria saves Kinji's life and insists he become her partner after witnessing his true capabilities.