In a world in which every corner has been explored, the Abyss remains the sole unexplored cavern. In this gigantic pit, the depth of which is unknown, weird and wonderful creatures live and valuable relics rest, the likes of which humans couldn’t dream of making. The wonders of the Abyss lure people to it, with many rushing to explore it. The adventurers who take up this challenge many times became known as Delvers or Cave Raiders.

Riko is an orphan who lives in Orth, a town built on the edge of the Abyss. She dreams of becoming a great Delver like her mother, and solving the mysteries of the Abyss.

One day, Riko discovers her mother Lyza’s White Whistle and decides to dive down into the depths of the Abyss. Along the way, she picks up a robot named Reg who suffers from memory loss, and he decides to follow her in hopes of recovering his memory. In the 4th layer, they are attacked by an Orb Piercer and Riko gets stabbed by one of its venomous needles. The poison starts spreading, but she is saved by the Hollow, Nanachi. Nanachi joins their party and the three of them continue on their adventure to the 5th layer, where Bondrewd awaits. It is there where they meet Prushka, a girl who was raised by Bondrewd and knows nothing of the outside world. She is fascinated by the tales of Riko’s adventures and wants to accompany her. Unfortunately, she is tragically used by Bondrewd in one of his experiments. However, while undergoing the experiment, Prushka transforms into Riko’s White Whistle and the party continues on their journey, delving deeper to the 6th layer, known as “The Capital of the Unreturned.” There, they discover a village of Hollows, whose inhabitants have a unique set of values.