The story begins with the protagonist, Akume Yuma, a veteran office worker of eight years, working late in his mundane city office. While reviewing documents, he discovers a peculiar contract with no client listed. The contract outlines terms for him to become a manager in another world due to a labor shortage, offering preferential treatment as a special hire and the ability to have one assistant. It also states he cannot unilaterally break the contract and must first sign his name. Dismissing it as a prank by a colleague, Yuma signs it.
Suddenly, the contract glows, and a striking, blue-skinned individual with golden hair and a face painted blue appears. This being thanks Yuma for finally accepting the contract, revealing that Yuma is the 50th person they've approached. They explain Yuma will become a manager in another world and offers him a choice of an assistant: a young girl who loves him or a muscular man who loves him. Yuma, bewildered, chooses the girl. With a snap of their fingers, a white pentagram appears on the floor, and a beautiful, dark-red-eyed girl named Eliel materializes, pledging her life and soul to Yuma. The blue-skinned being, revealed to be a demon, informs Yuma that his body will be transformed into that of a "demon" and that he will be a "Dungeon Master," an enemy of humanity. Yuma's consciousness then fades.
Yuma awakens in a cubic cave, finding Eliel, naked and apologetic for not being a "pervert." Eliel explains that dungeons generate energy from "maso," a unique particle, and that their location is unknown. She also reveals that dungeons are typically found in forests or mountains, but a city's proximity is dangerous due to adventurers. Yuma discovers he can create objects by sheer will, manifesting a bikini and then a maid outfit for Eliel, much to her surprise and delight. He also learns he can remodel the dungeon itself, transforming the cave into a stylish, izakaya-style restaurant with a hidden kitchen and living space.
Eliel reveals that Yuma's body has been fundamentally altered to be "top-tier" and that he is now a Dungeon Master, an entity perceived as humanity's enemy due to legends of Dungeon Masters allying with Demon Lords. Yuma also learns he cannot leave the dungeon physically, as doing so would cause his body to disintegrate, but he can project his consciousness into a created body to explore the outside world. He creates a handsome, young male body for himself and ventures out into the bustling capital city of the Rysels Kingdom.
In the city, Yuma witnesses a slave market where he encounters two high-priced female slaves, Remia and Charlotte, who dismiss him as a child. He then returns to his dungeon, where he and Eliel prepare to open their restaurant. Yuma, with his newfound ability to create anything he imagines, crafts elaborate dishes like burgers, BLT sandwiches, and beer, which are a massive hit with their first customers, two beastmen. Eliel, initially shocked by Yuma's immense power and the unusual nature of his creations, begins to adapt, though she expresses concern about the potential danger of their location in the capital's back alley.
Yuma continues to expand his dungeon, creating a vast, maze-like labyrinth beneath the restaurant, designed to buy them time. He also establishes a safe haven within the dungeon. Eliel, overwhelmed by Yuma's capabilities, struggles to keep up with his rapid progress. The narrative ends with Yuma contemplating the nature of his new existence as a Dungeon Master, his inability to leave the dungeon, and the potential dangers that lie ahead, while Eliel expresses her unwavering loyalty and willingness to follow his lead.
