Masuda Keima, a lazy high-school boy who prefers to laze around and avoid working, gets summoned as a dungeon monster by a girl avatar named "Dungeon Core No. 695" who is the dungeon core of the mighty one-room dungeon "Ordinary Cave".
Through a mishap she accidentally assigns him as her Dungeon Master, so first order of business is to get rid of those pesky bandit invaders that have set up camp near the heat-emitting dungeon core before they get the idea to smash it and kill them both.