OVA: Butei's Going on a Hot Spring Training Camp

Butei's Going on a Hot Spring Training Camp (武偵が着たりて温泉研修) is the OVA to the Hidan no Aria anime. It was released on December 21, 2011, included in the Hidan no Aria Blu-ray and DVD 7th Bullet volume

Synopsis
Umeko Tsuzuri takes Kinji, Aria, Shirayuki, Riko, Reki, Jeanne, and Muto to a secluded mountain onsen run by a former Butei for special training. During their long hike to the lodge, Aria, Shirayuki, and Riko fight over Kinji. While in the hot springs, the girls compare their chest sizes, much to Aria's dismay. Aria and Jeanne later have a ping-pong match using their special abilities.

Later, Muto is frightened upon seeing Jeanne temporarily turned into a chrysanthemum statue. She admits to losing memory between leaving the game room and ending up outside. Muto disappears that night, and the others later find him dead. Shirayuki then notices Tsuzuri and the landlady have been hanged. The group then spots a mysterious young boy and chases after him inside a haunted room. All of a sudden, the students find themselves back inside the minivan just before they arrive the inn.

While each member of the group remembers what happened up to their individual 'deaths', the landlady and everyone else not in the group do not recall anything and think the group are acting really strangely. Yet, the knowledge that the group learned from their mutual experience, such as events from the landlady's past, seems to be true. What exactly happened to the group remains a mystery.

Characters

 * Kinji Tohyama
 * Gouki Mutou
 * Aria H. Kanzaki
 * Riko Mine
 * Shirayuki Hotogi
 * Reki
 * Jeanne d'Arc
 * Umeko Tsuzuri

Trivia

 * This OVA is non-canon from the series
 * Seems to reference the japanese movie the grudge