The mysterious Abyss is a dangerous and cruel environment. Despite the risks, explorers like Riko, who aspires to follow in the footsteps of her mother as a Cave Raider, are drawn to it. Riko is shocked to learn that her mother has died in the Abyss, but a strange note informs her that she still may be alive somewhere down there. Riko sets off on a quest with Reg, a humanoid robot kid, determined to learn the truth about her mother's whereabouts. When so many others have perished, the two of them will see if they have what it takes to make it.