凶悪化する犯罪に対抗するため、武力で問題の解決にあたる探偵、通称「武偵」。
遠山キンジは、武偵の育成施設である武偵高校の生徒だったが、とある事件が元で、武偵高をやめて一般の学校に移るつもりでいた。
そんなある日、なぜかキンジは登校中に爆弾魔に命をねらわれてしまう。
だが追い詰められたキンジの前に、突然空から美少女が舞い降りてきた。
彼女の名は神崎・H・アリア。
小柄で可憐な外見からは想像のつかないような優秀な凄腕武偵で……狂暴で攻撃的な女の子だった。
この出会いが、キンジとアリア、2人の今後を大きく変えていく。
Hidan no Aria (Aria the Scarlet Ammo): In response to the worsening crime rate, Japan creates Tokyo Butei High, an elite academy where "Butei" or armed detectives hone their deadly skills in hopes of becoming mercenary-like agents of justice. One particular Butei is Kinji Tooyama, an anti-social and curt sophomore dropout who was once a student of the combat-centric Assault Division. Kinji now lives a life of leisure studying logistics in order to cover up his powerful but embarrassing special ability. However, his peaceful days soon come to an end when he becomes the target of the infamous "Butei Killer," and runs into an emotional hurricane and outspoken prodigy of the highest rank, Aria Holmes Kanzaki, who saves Kinji's life and demands that he become her partner after seeing what he is truly capable of.
