Set in an alternate 19th century, the story focuses on an organization of exorcists, named the Black Order, as they defend humanity against the Noah Family, reincarnations of Noah and his twelve apostles whom bear hatred towards humanity and God led by a man known as the Millennium Earl. The exorcists' main weapon against the Noah Family are sentient holy artifacts called Innocence. Innocence comes in a variety of forms, varying from everyday objects such as boots to grandfather clocks, to weapons such as swords and guns; regardless of their form, each Innocence possesses unique offensive and supportive abilities and will only work for the wielder of their choosing. Out of the 109 Innocence hidden and scattered throughout the world, one of them is the master Innocence; whichever side obtains this Innocence first will win the war. In contrast to the Innocence, the Noah Family's weapons are derived from a power source known as Dark Matter. Dark Matter, grants the Noah superpowers, along with the ability to create and control demons.
The central character is Allen Walker, a new recruit to the Black Order who started training to control his Innocence after it destroyed the Akuma of his late guardian, Mana. The story begins in a villain of the week fashion, where Allen teams up with various members of the Black Order to search for Innocence while battling Noah's demons on the way. Later, Allen and his friends are ordered to track down exorcist General Cross Marian, Allen's missing teacher. Their search concludes with them stealing one of the Noah's transportation device, referred to as the Noah's Ark; this was made possible since Allen has been instilled the consciousness of Nea, the exiled 14th member of the Noah Family who the Earl wishes to have back. During the Third Exorcists insurrection story arc, Nea's consciousness begins superseding Allen's body. Now hunted by the Black Order, the Noah Family, and a humanoid Innocence called Apocryphos, Allen goes into hiding as he searches for a way to end Nea's resurrection.
Information Source: Wikipedia