The great demon Mao Dante, exploiting the weakness of a boy exposed to his demoniac influence in the dreams, is awakened from a thousand years sleep.
The will of this being, a survivor of an ancient and forgotten civilization , is to wipe out the whole human race.
Anyhow nothing is so simple: the motivations of the demon aren't soo wicked as one could think and they will combine with the life of the boy which hangs between nightmare and craziness.
In the meanwhile the satanist will 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 it's remake in 2002. Nagai took many ideas from Mao Dante into Devilman.