Since the discovery of fantastical dungeons all across Earth, slaying monsters and clearing out dungeons has become a popular pastime. Twenty-seven-year-old Karaboshi Haruki dreams of gaining recognition as an adventurer, but he's got two monstrous problems: he's socially invisible and has literally no skills at adventuring. When a dungeon suddenly appears in his garage, however, he finds a skill board that allows him to level up faster. The dungeon-diving adventures Haruki has always dreamed of are just getting started!