D.Gray-Man depicts a young Exorcist named Allen Walker who has a cursed eye that can see the suffering souls of the Akuma Created by the Earl of the Millenium.

He joined the Black Order, the Exorcist Headquarters at England.
The battle with the Earl & the Ancient Noah Family  VS The Exorcists to save the world from
getting devoured by the Earl.

The Earl's plans were to make the entire human race turned into Akumas and the Exorcists
have the power of the Innocence that was given to them by God which allows that to kill
those Akumas and give them rest.

Watch/Read D.Gray-Man Anime or Manga to read how Allen Walker is going to save the world from getting dominated by Akumas, the ancient Noah Family and the Earl of the Millenium.