After bonding over a tragic loss, the long-suffering Nanachi joins Riko and Reg on their journey into the depths of the Abyss. Awaiting the children is the Sea of Corpses—the Abyss's fifth layer, and the deepest level from which a traveler can return without losing their human form.

The masked sadist Bondrewd stands between the children and the rest of their adventure. Bondrewd's horrific laboratory serves as a final checkpoint for those wishing to traverse deeper into the Abyss, and the sociopathic scientist has no desire to allow Riko's party to pass through at no cost. Deeply scarred by Bondrewd's impact on their childhood, Nanachi is engulfed in turmoil over his resurgence in their life.

Bondrewd's only apparent weakness is Prushka, a brash child who claims to be his daughter. Riko, Reg, and Nanachi befriend Prushka and work with the girl to overcome her father's machinations and breach the Abyss's sixth layer.