Hidan no Aria: Butei ga Kitarite Onsen Kenshuu
English title: Aria the Scarlet Ammo Special
Japanese title: 緋弾のアリア 武偵が来たりて温泉研修
Type: Special
Episodes: 1
Source: Light novel
Genres: Action, Comedy, Mystery, Romance
Studios: J.C.Staff
Producers: flying DOG
Duration: 23 min
Rating: PG-13 - Teens 13 or older

The story takes place in Tokyo Butei High School, a special school where armed detectives—"Butei"—are trained to use weapons. Kinji Tooyama 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 H. Aria Kanzaki, the most powerful S-Rank Butei student in Assault Studies.

(Source: ANN)