Japan establishes Tokyo Butei High, an exclusive institution for skilled "Butei" or armed detectives, as a response to the escalating crime rate. Within the school, Kinji Tooyama stands out as a withdrawn and brusque sophomore who used to study in the combat-driven Assault Division. Kinji chooses a tranquil existence studying logistics to conceal his potent yet humiliating ability. Despite his desire for a peaceful life, Kinji's tranquility is disrupted when he becomes the target of the dreaded "Butei Killer." In his moment of peril, Kinji crosses paths with Aria Holmes Kanzaki, an outspoken and brilliant prodigy of the highest level. Aria rescues Kinji from danger and demands that he join her as a partner, having witnessed his impressive capabilities.