Demons exist and their power is beyond what humans can fathom. In fact they are so strong that humans do not stand a chance against them in a fight. The only thing strong enough to defeat a demon is another demon and it is through this logic that Asuka Ryou hatches the plan to have his good-hearted friend Fudou Akira possessed by a demon. If a person has a pure heart, then he might be able to control the demon that possesses him and thereby acquire power equal to that of a demon. After raising some hell in a club the plan works and Akira is possessed by the powerful demon known as Amon. Now Akira is humanity's sole champion of justice against the hidden demon menace that has plagued mankind since the dawn of time.