In the Labyrinth, a vast underground realm, human colonies have long forgotten the surface world, which may now be nothing more than legend. To survive this harsh environment, colonists take on specific roles: Workers mine essential minerals, while Markers explore the unexpectedly lush Labyrinth to gather data for navigation. Humanity constantly fights kaijuu, monsters ranging from tiny to colossal, all of which are deadly. Some kaijuu can even breach colony walls, increasing the danger.
Memenpu, a nine-year-old college graduate, has created innovations that greatly help the Workers in her colony. She dreams of a place without boundaries, a sky stretching endlessly above. Inspired by these visions, Memenpu wants to become a Marker and uncover the Labyrinth's mysteries. Her father, Gagumber, opposes this ambition, but a sudden encounter with a kaijuu triggers an eventful, dangerous journey that could change humanity's future.