"The Abyss", a mysterious colossal pit that is inhabited by many kinds of amazing creatures. An energetic girl called Riko dreams to become a great Cave Raider as her mother, and thus she starts the adventure to the deepest layer of The Abyss with Reg, a robot that wants to get back his memory. Together they descend into the Abyss' treacherous fifth layer, they encounter Bondrewd and his daughter Prushka. During the adventure, Riko and Reg discover the secret of The Abyss...