Unaired OVA bundled with the game Bakumatsu Rock: Ultra Soul . 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)
Watch Bakumatsu Rock: Mystery! Onsen Kaijiken ze yo!! episode 1 online — subbed and dubbed, with automatic AniList / MyAnimeList progress sync.