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 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 they might be able to control the demon that possesses them, 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 humanity's sole champion of justice against the hidden demon menace that has plagued humanity since the dawn of time.
 
(Source: Anime News Network)

Akira is just another normal kid in modern-day Tokyo, until an old friend of his, Ryo, shows up and turns his world upside down. Akira learns that there is an upcoming war of demons on humanity and he has just been enlisted for a major tour of duty. But the only way to fight a demon is with their power, so Akira and Ryo risk a dangerous ceremony in an attempt to create humanity's only hope: the powerful Devilman. 

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. 

Hordes of demons, once thought entombed forever in the underworld, have now returned to Earth. Evil and corruption begin to seep into our world, as demons seize human hosts. Mankind’s only hope for salvation is to use the demons’ power against them–and only a pure-hearted man like Fudo Akira can do so without losing his humanity. Once little more than a crybaby, Akira now wields the terrible power of a devil, yet holds the innocent soul of a man–Devilman! 
 
(Source: Seven Seas Entertainment)

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