Dungeon ni Deai wo Motomeru no wa Machigatteiru Darou ka OVA

Is It Wrong to Try to Pick Up Girls in a Dungeon?: Is It Wrong to Expect a Hot Spring in a Dungeon?

Following the crisis on the dungeon's 18th floor, Bell Cranel and his allies head back to the surface. However, their journey is delayed midway when the group stumbles across uncharted territory that hides a pleasant surprise—a hot spring! The group takes a respite from all the hardships they have endured. Unfortunately, as everyone relaxes in the bath, they discover that the hot spring may hide an evil secret. [Written by MAL Rewrite]