Beneath the surface of the earth lies the Abyss, a massive chasm filled with bizarre creatures and ancient relics from forgotten times. The mystery of its origin and the secrets it holds have drawn countless explorers, known as Divers, to descend into its depths at tremendous risk. Among these, the most revered and brave are the White Whistles, celebrated as heroes by those who fear the downward journey.
Riko, whose mother is the legendary White Whistle Lyza the Annihilator that disappeared during an expedition, dreams of reaching the Abyss's deepest levels. Currently, she is only a novice Red Whistle, restricted to the uppermost layer. One day, she discovers a peculiar robot resembling a young boy who has no memory of his past. Naming him Reg, Riko suspects the advanced technology within him must come from the depths of the Abyss. Together, they embark on a journey into the abyssal darkness: Reg searches for his lost memories, while Riko is resolved to reach the bottom herself. However, they remain ignorant of the brutal realities that await them in the unforgiving world of the Abyss.