The plot, which takes place at the end of a fictional 19th century Europe, revolves around Allen Walker. Allen is a member of the Black Order, a group of Exorcists connected with the Vatican. Their mission is to stop The Millennium Earl, an evil being planning to cleanse the world by destroying all the humans in the manner of Noah's great flood.
The Exorcists themselves are special people gifted with the ability to control and use Innocence, a divine substance created in the ages past to combat the Earl's minions, the Akuma. It is said that there are exactly 109 units of Innocence scattered around the world. Once an Innocence finds its compatible person, it will evolve to become a weapon to fight the Akuma.