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, filled with monstrous creatures and lost relics. Those who delve into its depths 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 while exploring the Abyss, she is one perilous step closer to achieving her goal!
Riko awakens in the sixth layer, but Reg and Nanachi are nowhere to be found. Riko's search for her companions takes her deep into the Village of the Hollows, where she must make a decision that could mean the end of Nanachi''s freedom--or the end of Riko's life.