The Abyss is an immense and precarious expanse that plunges into the depths of the earth, teeming with perplexing beasts and artifacts from times long forgotten. The origin of this abyssal marvel and the secrets hidden in its deepest recesses have intrigued many brave explorers known as Divers, who have risked their lives to uncover its mysteries and confront the unknown dangers lurking below. Among these audacious adventurers are the legendary White Whistles, hailed by those above ground as heroes. Riko, daughter of the vanished White Whistle Lyza the Annihilator, yearns to emulate her mother's footsteps and delve into the farthest corners of the Abyss. However, as a novice Red Whistle, she is restricted to the shallowest layer. While exploring the shallows, Riko meets a curious amnesiac boy and christens him Reg, convinced that his origins lie further within the Abyss. Together, they embark on a perilous descent into the depths of the abyss, hoping to unveil the secrets of Reg's past and witness the enigmatic bottom of the pit. Unbeknownst to them, the Abyss is an inhospitable and treacherous place, brimming with fatal hazards and bewildering wonders far beyond their imagination.