Devilman starts with a group of Angels descending down onto the Cretaceous era of Earth where they are suddenly attacked by an armada of demons, led by an angel traitor Satan. The Angels flee and merge together and summon God who attempts to eradicate the demons; however, they go into hibernation deep in the Himalayan Mountains.

The story then follows a boy in his late teens named Akira Fudo, who after unexpectedly encountering his old friend Ryo Asuka is thrown into the world of demons. Ryo takes him to his home where he tells Akira about his Father's recent death and how he had been researching demons and that he wanted Akira to join him in has battle against them. To do this he needed Akira to risk his own life to fuse with a demon. Akira agrees and Ryo takes him to a Black Sabbath where Akira successfully merges with the demon Amon, who then goes on to slaughter the other demons who had merged with the partiers at the Black Sabbath. However, Ryo is seriously injured in the battle.