The Abyss—a vast pit diving deep into the earth, is home to odd creatures and ancient relics. The origins of this immense chasm and what rests at its base remain a mystery, compelling many courageous explorers, known as Divers, to delve into its shadowy depths. Among these explorers, the elite White Whistles, celebrated as legends on the surface, represent the pinnacle of bravery.
Riko, whose mother, the renowned White Whistle Lyza the Annihilator, went missing, dreams of emulating her mother’s achievements and exploring the deepest parts of the Abyss. Currently a novice Red Whistle, Riko is restricted to the shallowest layer of the Abyss. Despite this, her life takes a turn when she encounters a mysterious robot that looks like a young boy, whom she names Reg. Reg has no memories of his past, leading Riko to believe that the technology behind his creation must originate from deep within the Abyss. Driven by this belief and a desire to unveil the depths of the pit, Riko and Reg set out on a journey into the Abyss to recover his lost memories and witness the true nature of its depths, unaware of the harsh realities that await them.
[Adapted from MAL]