Originally a "gothic" manga/anime that takes place at the end of "an alternate 19th century." It tells the story of a young boy named Allen Walker, and his friends, the other Exorcists and those who support them.

All Exorcists are members of the Black Order, a secret religious organization dedicated to stopping the Millennium Earl. The Earl is a crazed ancient who desires to "cleanse the earth" by destroying all of mankind - with the assistance of Akuma, demons formed from a mix of machine, a despairing soul, and the corpse of the mourned. At his side at the descendants of the biblical Noah, supernatural humans with the ability to switch between "white" and "black" forms.