An enormous pit and cave system called the "Abyss" is the only unexplored place in the world. Strange and wonderful creatures reside in its depths, and it is full of precious relics that current humans are unable to make. The mysteries of the Abyss fascinate humans, and they head down to explore. The adventurers who venture into pit are known as "Cave Raiders." A little orphan girl named Rico lives in the town of Ōsu on the edge of the Abyss. Her dream is to become a Cave Raider like her mother and solve the mysteries of the cave system. One day, Rico starts exploring the caves and discovers a robot who resembles a human boy.