The Abyss – a mysterious hole that descends deep into the earth. The town of Orth surrounds it, and many artefacts and relics are discovered in its depths by hunters called Cave Raiders. An orphan girl Riko, longs to descend the depths following the footsteps of her mother Lyza, a legendary Cave Raider. One day in the upper depths, Riko discovers a robot resembling a human boy and names him Reg. Determined to find her mother after evidence of her existence rises to the surface, she takes Reg as her companion and together they descend the depths of the Abyss.