In a distant future where humanity has abandoned a now-inhospitable surface, the remnants of civilization live in cramped, dangerous underground colonies known as "圈落" (Koraku). These settlements are scattered throughout a vast, uncharted, and cavernous frontier called the Labyrinth. Life underground is a constant struggle against monstrous creatures known as Kaiju and the ever-present threat of collapsing tunnels. The most dangerous yet vital profession in this world is that of a Marker, an explorer who ventures into the Labyrinth to map its treacherous passages and secure resources for survival.

The story centers on a mismatched father-daughter duo. Memempu is a precocious and fiercely curious nine-year-old who has already graduated from college. Tormented by recurring dreams of a mysterious white tower standing under an open sky—a concept of a "surface" that many believe is a myth—she is determined to become a Marker and find this legendary place. Her father, Gagumber, is a rough-and-tumble manual laborer, a "Worker" who operates a heavy-duty mecha to mine ore. A former legendary Marker known as the "Gale-force," he is now a cynical, lazy, and overprotective single father who wants nothing more than to keep his daughter safe from the lethal dangers of the Labyrinth, leading to constant, loud, and inventive bickering between them.

Their stagnant life in the colony of Pinyin is shattered when a devastating Kaiju attack, combined with a mysterious pendant sent by a legendary Marker named Urorop, forces Gagumber to relent. The pendant contains a map pointing toward the location of Memempu’s dreams. Climbing into their two-person work robot, "Big Tony," the father and daughter embark on a perilous journey across the Labyrinth. Their quest is not just a physical traversal of monster-infested caverns but also an emotional journey to bridge their clashing worldviews.

As they travel to various unique colonies, their party expands. They are reluctantly joined by Zackletu, a cool, motorcycle-riding, and money-obsessed female Marker who harbors a complicated grudge against Gagumber from their shared past. They also encounter Yuri, a hot-headed but good-natured young hacker who leads a group of orphans fighting against systemic corruption. Together, they face numerous arcs that define their journey: navigating the politically rigid colony of Jolly-Jolly, unearthing the dark secrets behind the seemingly utopian colony of AreYaar, and confronting Gagumber's traumatic past, including the death of his former partner, Rufus.

As the group delves deeper, they uncover the truth about the Labyrinth’s origins and the nature of the surface world. The journey forces Gagumber to confront his failures and reclaim his lost skills to protect his daughter, while Memempu must reconcile her idealized vision of adventure with the harsh, often heartbreaking realities of the world. Their expedition ultimately evolves from a simple treasure hunt into a race against a conspiracy that threatens the very foundations of their underground civilization.