Demons are real, and their strength is far beyond human comprehension. In fact, they are so formidable that humans have no chance of defeating them in combat.
The only force capable of overcoming a demon is another demon, and based on this reasoning, Ryo Asuka devises a plan to have his kind-hearted friend, Akira Fudo, taken over by a demon. If a person has a pure enough heart, they might be able to control the demon that possesses them, thus gaining power equal to that of a demon. After causing some chaos in a nightclub, the plan succeeds, and Akira is taken over by the mighty demon named Amon.
Now, Akira stands as humanity's sole champion of justice against the hidden demon threat that has plagued mankind since the beginning of time.