The enormous cave system, known as the Abyss, is the only unexplored place in the world. Nobody knows how deep down goes this titanic pit, inhabited by strange and wondrous creatures and full of mysterious ancient relics which purpose in unknown to modern man. Generations of bold adventurers have been drawn by the cryptic depths of the Abyss. In course of time those bold enough to explore the dangerous pit came to be known as "Cave Raiders."In Oosu, the town at the edge of the Abyss, there lives a little orphan named Rico, who dreams of becoming as great a cave raider as her mother was and solving the great mystery of the pit. One day exploring the murky depths she stumbles upon a little boy, who turns out to be a robot...
While exploring the Abyss, Riko comes across something she has never seen before. How does this new discovery impact her?
Riko and Reg find out more about their past but the question still remains, who or what exactly is Reg?
Reg goes on his first cave raid! Meanwhile, Riko makes preparations to go into the abyss to find her mother.
Riko and Reg start their adventure into the Abyss, while there they run into an unexpected familiar face.
Riko and Reg descend into the Forest of Temptation. What new dangers will they find laying in wait for them there?
Reg and Riko make it to the Seeker Camp where they meet Ozen the Immovable, the person who saved Riko all those years ago. But there is more to Ozen than meets the eye.
Ozen reveals more about the Abyss as well as the truth behind Riko's origins. What will happen when Ozen shows her true colors?
Ozen tasks Riko and Reg to survive alone outside the Seeker Camp. Do they come out alive and unscathed? Or does the Abyss have a dirty trick up its sleeve?
Reg and Riko continue their descent after they leave Ozen's Seeker Camp. Riko's mettle is tested as Reg passes out and she has to fend for herself.
Reg and Riko run into trouble deep in the fourth layer where they meet an interesting creature, but is she friend of foe?
Nanachi sends Reg on a mission to find certain items that could possibly save Riko. Meanwhile, Nanachi reveals a little more about themself.
Nanachi shows Reg the truth about the Abyss. Meanwhile, Riko is still recovering from her injuries when she has a strange dream.
Nanachi and Mitty's past is revealed. How did they become what they are and who is responsible for it? Meanwhile, Riko is on the mend after her injuries.