The underground cave system known as Abyss is the last largely unexplored place on this planet. No one knows how deep the gigantic chasm goes, which is inhabited by wondrous creatures and where countless mysterious relics can be found. The orphan girl Riko has set herself the goal of following in her mother Lyza's footsteps and, as a cave diver, uncovering the legendary mysteries of the cave system. She is supported by the robot boy Reg and other companions. After a fierce battle against the brutal Lord Bondrewd, the group finally reaches the sixth depth layer. However, they soon realize that their lives and their mission are once again at stake.