The distant future, long after the Western calendar is no longer used. Humans live shoulder-to-shoulder, cramped in the "Labyrinth," an underground world deep below the surface. There are many colonies where people live in the underground Labyrinth. The place is known for extreme high temperatures, but also for lodes of gold, silver, and other riches. In one of the colonies named Pin-in, a curious nine-year-old girl named Memenpuu and her father Gaganbaa board a two-person work robot and mine for ore. They eke a living as the lowest of lowest "Worker" occupations.

One day, Memenpuu begs her father Gaganbaa to set out as Markers — those who chart the innards of the Labyrinth as a spelunker — to search for her mother who left their home. If they encounter a "mysterious giant creature" lurking in the caves, the survival rate is less than 5%. Since discovering an unknown world is dangerous work, "maps" hold great value in the world of Labyrinth and fetch high prices on the market. A Marker is the most dangerous, but also the most lucrative, job. Gaganbaa decides he cannot hold back the ever-curious Memenpuu from going off on her own countless times, and finally gives in. The two embark on a journey together in the work robot.

However, the "cost" of curiosity... The "truth" they finally uncovered...

This is a father and daughter's epic journey with the theme of family love.

(Source: Anime News Network)