In Dawn of the Deep Soul, the brave Riko and Reg, accompanied by the new addition to their team, Nanachi, embark on an epic adventure that leads them to the perilous fifth layer of the Abyss, known as the Sea of Corpses. Here, they encounter the mysterious Bondrewd, a renowned White Whistle whose existence has troubled Nanachi. Though Bondrewd appears friendly and hospitable, the trio remain wary of the Abyss's concealed threats and potentially lethal dangers.