Akari Mamiya is an Rank E freshman who looks up to her senior, the S-ranked Aria H. Kanzaki. They both attend Tokyo Butei High School, a school that trains its students as Armed Detectives otherwise known as “Butei” to society. Individual student training aside, the school employs the co-operative “Amica” system, allowing an underclassman to train alongside an upperclassman. Akari is determined to be paired with Aria.