Yuki wakes reincarnated as a demon lord, bound to a single dungeon in a world where powerful monsters roam free. He expands and defends it with the DP Catalog, an in-world shop that sells him floors, traps and summons, just to stay alive. The dungeon fills up fast: Lefisios, the supreme dragon, who has a sweet tooth and no intention of leaving, a small vampire girl named Iluna, and a slime he keeps as a pet. Days pass more in housekeeping than in battle.