Riko, Regu, and their newfound companion Nanachi continue their deadly trip below the Abyss, reaching the fifth layer, The Sea of Corpses. When the core three arrives to the Idofront research facility, they meet the enigmatic Prushka, the supposed daughter of Bondrewd, who brings them to the White Whistle, who is responsible for Nanachi's tragic background. Despite the friendly looks of the people of Idofront, Nanachi warns the young explorers that not everything is as it appears. What sacrifices must be made in order to continue the trek down to the bottom of the Abyss, with the sole path to the sixth layer cloaked in mystery and Bonedrewd's plots awaiting them?