"Abyss" is the only unexplored area left in the world where everything else has been explored.
In the vertical hole which depth is unmeasured, very strange monsters live and the precious ruins and artifacts, which the humans cannot make with their technology, are stored.
Mysteriousness of "Abyss" charms the people, leading them to adventure.
The adventurers who continually challenge the big hole are becoming known as the "Cave Raiders."

Riko is an orphan who lives in the town "Oath", which is located on the edge of the Abyss, and dreams of becoming a great Cave Raider like her mother and explores the secrets of the Abyss.

Riko is determined to go deep into the hole after the white whistle of her mother, Lyza. Amnesia robot Reg, who gets fixed by Riko, also decides to go into the hole together with Riko, in order to bring back his memory.

Riko suffers from the poison of Tamaugachi on the 4th floor.
Riko is rescued by the ruined existence, Nanachi.
Adding Nanachi to her party, the 3 adventurers steps on the 5th floor where Bondrewd awaits.
There, they meet a girl called Prushka...