Tokyo Butei High School is a special school where armed detectives—"Butei"—are trained to use weapons. Kinji Tōyama is a second year student who has a special ability, but he keeps it a secret to maintain an ordinary, peaceful life. However, when he gets caught in a bombing on the way to school, he encounters Aria H. Kanzaki, the most powerful S-Rank Butei student in Assault Studies.