A young boy is entrusted by a god to be a demon lord and given a dungeon. With various blessings from the god, he is ready to start his demon lord life... but due to the god's careless mistake, he ends up being the weakest demon lord in the world. To survive, he continues to build his dungeon day by day. This is a story about a slightly silly protagonist, a lonely demon child, an undine living in a bath instead of a lake, and princesses who fall in love with the demon lord while taking baths together.