The great demon Mao Dante, exploiting the weakness of a boy exposed to his demonic influence in his dreams, is awakened from a thousand-year sleep. The intent of this being, a survivor of an ancient and forgotten civilization, is to wipe out the whole human race.
But it's not that simple: the motivations of the demon aren't so wicked as one might think, and they will combine with the life of the boy, which hangs between nightmares and craziness. In the meanwhile, the Satanists fight against the hierarchy of the church to obtain a justice which is, as it's easy to guess, that of the demon Mao.
This series didn't have an ending until its remake in 2002. Nagai took many ideas from Mao Dante into Devilman.