The "Labyrinth" is a vast underground space where humans reside in groups called "colonies." Over time, the surface has faded into distant memory—or perhaps remains merely a fantasy for those who have never witnessed its wonders. To ensure humanity's survival in the harsh underground conditions, colony citizens undertake various specialized roles. These include "Workers," who mine precious ore to power the colonies, and "Markers," who venture into the Labyrinth's unexpectedly lush environment to gather information that facilitates navigation. However, humanity also faces an existential threat from creatures known as "kaijuu," which range in size from that of a small child to a massive building and are hostile toward any human they encounter. Furthermore, kaijuu of sufficient size can breach the colonies, thereby escalating the danger. Memenpu is a nine-year-old college graduate whose inventions have significantly aided the Workers in her local colony. Lately, however, she has been dreaming of a place with an endless ceiling, unbounded by bedrock. These aspirations drive her ambition to become a Marker and explore the Labyrinth's vast unknown in search of such a fantastical location. Despite her father Gagumber's strong opposition, a specific incident involving the kaijuu triggers a perilous yet thrilling adventure that will undoubtedly change the course of humanity forever.