After the demon lord's death, the remaining demon race repurposes the abandoned dungeons into commercial ventures, creating a world where "commercial dungeons" are a booming business. Fran, a young commercial dungeon designer, struggles with repeated failures in her work. Discouraged, she encounters a small slime that claims to be the former demon lord who once built countless dungeons. Intrigued by its knowledge and experience, Fran teams up with the slime to design and operate successful dungeons. Together they form the "Foot of the Cave" team, aiming to create the most engaging and profitable dungeon experiences. The story follows their partnership as they navigate challenges, competition, and the evolving dungeon industry. A major narrative arc is the "Great Dungeon Expo", a quadrennial event where the best dungeons compete for the prestigious "Dungeon of the Year" award. Their main rival is the veteran designer Cynthia, who leads the immensely popular "Tomb of the Dead" dungeon and develops an obsessive fixation on Fran. Later, Cynthia's father, a figure who despises commercial dungeons, stands in their way, threatening the very existence of their business and forcing Fran and the slime to fight for the future of commercial dungeons.