Clusters of human colonies thrive within the underground "Labyrinth," rendering the surface above an illusion or distant memory for the unacquainted. The successful functioning of this austere society relies on specialized professions, such as miners and explorers called "Markers," who traverse the deceptively exuberant landscape and furnish valuable data. The populace of this place is, however, continuously challenged by aggressive creatures named "kaijuu," which can vary from small to colossal enough to infiltrate colonies. Memenpu, a nine-year-old college graduate, devises cutting-edge machinery that substantially supports the Workers in her nearby colony. Nonetheless, she yearns for a limitless sky unencumbered by rock and aims to become a Marker and unveil this mythical realm. Even as her father vehemently opposes her dreams and she faces danger from the kaijuu, Memenpu inexorably embarks on an adventure that modifies humanity's future irrevocably.