Yuki dies and wakes up in another world as a demon lord with a dungeon of his own, surrounded by monsters strong enough to kill him without effort. Rather than panic, he sets about building up its defenses and filling it with company: first Lefi, the supreme dragon who sits at the top of the food chain, then Iluna, a vampire girl who moves in after her home is destroyed. Between monster raids and adventurers who come to clear the place out, his new life as a demon lord turns out to be surprisingly cozy.