In an age when the corners of the world have been scoured for their secrets, only one place remains unexplored–a massive cave system known as the Abyss. Those who traverse its endless pits and labyrinth-like tunnels are known as Cave Raiders. A young orphan named Riko dreams of following in her mother’s footsteps as a Cave Raider, and when she meets a strange robot when exploring the Abyss, she’s one step closer to achieving her goal!