When entrusted with a dungeon by God, a boy takes on the title of Demon Lord. However, due to a divine blunder, he turns out to be the weakest Demon Lord in existence. To survive, he must keep creating dungeons. This tale follows a foolish main character, a solitary demon child, a bathtub-dwelling Undine, and a princess who defies norms by falling for the Demon Lord despite her human nature. All these events transpire in the most unexpected of settings – during a bath.