Young Riko has spent her entire childhood dreaming of exploring the Abyss, an enormous pit and series of caves filled with wonders and terrors that only the staunchest Cave Raiders have survived to bring back ancient artifacts from its depths. No one knows just how deep the pit is, but Riko's own mother disappeared into it years ago, leaving her daughter behind with a powerful urge to follow after her and learn the Abyss's mysteries for herself. One day Riko discovers a boy who seems to have originated from the Abyss and appears to be a sort of robot. Can he aid the little girl in her quest to find her missing mother? Or will the children become the Abyss's next victims?