On a ruined far-future Earth of colossal tombs and decaying cities, a class of beings called Gaunas can grow weapons and armor out of bone-like matter. Kudou Denji is a Black Gauna who would rather be left alone, but the Observation Bureau, the government body that engineered his kind, presses him into hunting White Gaunas. Those are people who mutate into ravenous monsters and tear through what is left of human settlements. Reluctantly drawn into the fight, Denji becomes the line between the survivors and the creatures hunting them.