Akari Mamiya, a first-year at Tokyo Butei High, idolizes the legendary S-Rank Butei Aria H. Kanzaki, a fourth-generation descendant of Sherlock Holmes, and dreams of becoming an armed detective just like her. There is just one problem: Akari tests at E-Rank, the lowest in her class. When she talks her way into a trial set by Aria herself and passes it, Akari gets her shot at becoming Aria's Amica, the junior an upperclassman takes on to train, and at closing the gap between her rank and her hero's.