***The plot revolves around Allen Walker, an Exorcist, in the end of the 19th century Europe. 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 ghoul intending 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.
As the story progresses, Allen Walker is introduced to more Exorcist characters, among them are Yu Kanda, Lenalee Lee, Lavi, Arystar Krory, and Miranda Lotto. Also in the spotlight is the older brother of Lenalee Lee, Komui Lee, the church's science department supervisor and Bookman, a mysterious colleague within the church; and Cross Marian, a General in the Organization and Allen's master.
More villains also appear along the way. There are higher level Akuma, with greater power, skill and wit. Also appearing is the Clan of Noah, a group of humans descended from Noah himself. Each member is gifted with great power, and the group has allied themselves with the Earl.