The Abyss is a huge pit reaching down into the earth's depths, teeming with enigmatic animals and treasures from a long time ago. How did it come about? What is at the very bottom? Countless courageous individuals known as Divers have attempted to unravel the secrets of the Abyss by boldly plunging into its lowest depths. Those who remain on the surface revere the White Whistles, the greatest and bravest of the Divers. Riko, the lost White Whistle Lyza the Annihilator's daughter, wishes to be like her mother and explore the depths of the Abyss. However, as a new Red Whistle, she is only allowed to explore its uppermost stratum.