Keima Masuda's motto is to do nothing but laze around all day. This still rings true after he is summoned to a world of magic by Dungeon Core No. 695, nicknamed Rokuko, the dungeon master of a pathetic little room in a cave. Even worse, her home has been occupied by a group of bandits and Rokuko's few remaining dungeon points are only good enough for spawning a couple of goblins. Keima is only spurred into doing something when he is informed that he will die along with the little girl if her dungeon core is destroyed.


Although despairing at having wasted 1000 of her precious points on a frail human who sleeps all day, Rokuko soon appreciates that there is some level of ability in her new summon. Keima eliminates the immediate threats and slowly builds a thriving village centered on Rokuko's cave where unsuspecting human adventurers can farm monsters and level in peace, earning the dungeon a steady income of dungeon points. Through Keima's unconventional strategies, the duo develop their dungeon and their relationship, gradually rising through the ranks and gaining new allies and ultimately establishing themselves amongst the hundreds of other dungeon cores in the world.