Akira Fudou leads the life of a typical high school student. After a chance encounter with school bullies, he is rescued by his childhood friend Ryou Asuka. Ryou and Akira catch up over the course of an afternoon that ends with them crashing a party at a nightclub. Initially uncomfortable with the environment, Akira is shocked when violence provokes some of the club's partygoers to transform into demonic monsters.

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 trough this logic that Ryo Asuka hatches the plan to have his good-hearted friend Akira Fudo possessed by a demon. If a person is of pure of heart then he might be able to control the demon that posses him and thereby acquire power equal to 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 humanities sole champion of justice against the hidden demon menace that has plagued humanity since the dawn of time.

Fear runs rampant throughout Tokyo with the revelation that demons in fact exist amongst us. Paranoia and the darker side of humanity boils onto the streets as people turn on one another, suspecting that anyone could in fact be a demon hiding in human clothing. Amidst the growing tensions, tragedy strikes Akira causing his mind to snap, retreating into his subconscious, allowing his Devilish alter-ego Amon to break free from Akira's cage of flesh and wreak havoc on both human and demons alike.