A young boy is entrusted with a dungeon by God, to serve as a Demon Lord. Granted various blessings by God, he's all set for his life as a Demon Lord!! ...Or so it was supposed to be. Due to God's oversight, however, he becomes the world's weakest Demon Lord. To survive, the Demon Lord continues to build dungeons even today.

This is a story that unfolds as a somewhat goofy protagonist takes baths with a lonely ogre child, an Ondine who settled in a bathtub instead of a lake, and human princesses who, despite their humanity, fell in love with the Demon Lord.