The Abyss is a hole of unprecedented depth, where one young girl and a robot brave its dangers to find the truth. The town of Orth is unique, built around the edges of the massive Abyss, a wonder that has never been fully explored. Those who venture too far down never return, but those brave enough to traverse its territories are known as "Cave Raiders" and are heralded as legends. Living in this town is a young girl named Riko, the child of one of the most famous Cave Raiders of all time, who disappeared on an expedition many years ago. One day, Riko's life changes when she meets a strange robot called Reg, who seems to have appeared from within the Abyss. Believing this to be a sign from her mother, who is stuck at the bottom of the Abyss, Riko descends into its depths with Reg, ready to confront all the dangers within it.