Continuing their perilous descent down the Abyss, Riko, Regu, and newfound friend Nanachi reach the Abyss' fifth layer, The Sea of Corpses. Upon arriving at the research station known as Idofront, the main trio encounter the mysterious Prushka, the alleged daughter of Bondrewd, who leads them to the White Whistle responsible for Nanachi's dark past. Despite the welcoming appearances of Idofront's residents, Nanachi warns the young adventurers that things are not always what they seem. With the only route to the sixth layer shrouded in mystery and Bonedrewd's schemes awaiting them, what sacrifices must be made in order to continue the journey down to the bottom of the Abyss?

[Written by MAL Rewrite]