Zenon, the ruler of the demon world, orders the extermination of humans. However, Amon, one of the four heavenly demons, did not consider humanity to be as weak as the others did. This stance caused unease among the rest of the ranks, prompting Amon to flee them before encountering Kei Kamishiro, a junior high school student. The two strike a deal to cooperate by fusing into a new identity known as Devilman.