Despite being beyond human comprehension, demons possess unbeatable power in battle. However, Ryo Asuka has uncovered a way to defeat them - enlisting a good-hearted ally, Akira Fudo, to become possessed by a demon. With the belief that one's virtuous heart can control the demon within, Ryo and Akira successfully execute their plan after causing chaos at a club. Akira is now inhabited by the mighty demon Amon. Armed with his newfound demonic strength, Akira becomes humanity's only hope against the century-old demonic menace lurking in the shadows.