Bakumatsu Rock: Mystery! Onsen Kaijiken ze yo!!
Genres: Mystery
Type: OVA, Episodes: 1
Aired: Sep 25, 2014
Studio: Studio Deen
Source: Game
Rating: PG-13 - Teens 13 or older
Synopsis:
An incident has led to someone's death at Teradaya, which the gang went to for a day trip. A "69" on the victim's leg will be the key to solving the mystery.

(Source: MAL News)