Zenon, the ruler of the demon world, declares the extermination of mankind and begins an invasion of the Earth. However, one of the four heavenly demons, Amon, rejects humans as weak beings whom they don't need to fight, and attacks Zenon, only to be defeated by his overwhelming power. Amon is exiled from the demon world and continues to fight numerous assassins, and escapes to the human world with a mortal wound, where he meets a human boy, Kamishiro Kei. Kei is targeted by the demons for saving Amon, but despite his panic, he chooses to muster his courage and act as a decoy in order to help his friend escape. What will happen if his strong will is matched by strong power? Amon, intrigued, merges with Kei to save him, becomes Devilman, and throws himself into the battle against the demons.