It's been five years since strange dungeons opened up around the Earth, spilling monsters into our world. Average citizens everywhere have risen to the challenge to become adventurers, fighting these fantasy creatures and clearing out dungeons as best they can. Among them is Karaboshi Haruki, a 27-year-old rookie overlooked by everyone and less than successful at beating monsters and finding treasure. Then an opportunity drops into his lap: a dungeon suddenly appears beneath his home! The dungeon-diving adventures Haruki has always dreamed of are just getting started.