Akuma, monsters born from dead souls mistakenly recalled by their loved ones to the living world, are the weapons of the Millenium Earl, who is intent on the destruction of the world. Allen Walker has been cursed with an anti-Akuma weapon as his left arm and an eye that sees the former soul of the Akuma since his youth. He is also one of the few who contain "Innocence", a substance that allows him to kill Akuma and fight the Earl. With these powers, he joins the Black Priest Organization as an Exorcist and sets out with other Exorcists to stop the Earl's attempts to annihilate the human race.
The Millenium Earl has created a weapon called Akuma (monsters) to destroy mankind. The only people who can stop them are the excorcists from the "Dark Religious Organization"