High school student Maruo Anai is suddenly transported to a fantasy world along with his entire class. While his classmates receive powerful hero-class jobs, Maruo is assigned the seemingly useless Hole Digger job and is branded as a failed hero. Resigned to his fate, he begins digging a hole alone and accidentally discovers a newborn dungeon core. Touching it transforms him into a Dungeon Master, granting him the ability to expand and manage his own dungeon. Using his combined Hole Digger and Dungeon Master skills, Maruo focuses on building a comfortable underground home, mass-producing cute, fluffy monsters, planting crops, and enjoying a slow life far from the battlefield. However, his peaceful plans are disrupted when his dungeon, which he never meant to be dangerous, begins to automatically generate monsters far stronger than any hero. The authorities quickly label his creation a catastrophe-level threat. As the story progresses, Maruo reluctantly interacts with his former classmates, who are unaware of his true power, and his dungeon accidentally connects to another mysterious dungeon, leading to a confrontation with Emilia, a rival Dungeon Master with a grudge against Maruo’s first companion, the demon girl Azurine. The narrative blends dungeon-building and slice‑of‑life comedy with growing external tensions, as Maruo’s undesired reputation as the master of the world’s strongest dungeon only escalates.