In a world where the Abyss looms, a vast chasm teeming with enigmas and relics of yore, curious individuals called Divers plunge into its depths to unravel its secrets. Among them are the revered White Whistles, elite explorers of legendary status. Riko, daughter of the renowned yet absent Lyza the Annihilator, dreams of emulating her mother's legacy by delving into the Abyss's farthest recesses. While Riko, a novice Red Whistle, is restricted to the upper layers, her encounter with a peculiar robot resembling a young boy, christened Reg, alters her fate. With no memory of his origins, Reg and Riko embark on a perilous journey into the Abyss, convinced that his existence is tied to its unfathomable depths. Little do they realize the grim truths lurking beneath the surface, shrouding the Abyss in a veil of danger and mystery.