In order to find Lyza, the mother of the greatest white whistle, Riko deep into the world's remaining abyss, "Abyss", with the lost memory robot Reg, encounter various mysterious creatures and dangers along the way, she even accompanied Nanachi, who was hollow. However, at the bottom of the abyss, she met one of the legendary white Whistle "Bondrewd" and his daughter, but somehow discovered the secret of the abyss and the hollow...all the conspiracy came out...
