Here's the basic plot and story:
Akuma, monsters born from dead souls mistakenly recalled by their loved ones to the living world, are the weapons of the Millenium Earl, who is intent on the destruction of the world. Allen Walker has been cursed with an anti-Akuma weapon as his left arm and an eye that sees the former soul of the Akuma since his youth. He is also one of the few who contain "Innocence", a substance that allows him to kill Akuma and fight the Earl. With these powers, he joins the Black Priest Organization as an Exorcist and sets out with other Exorcists to stop the Earl's attempts to annihilate the human race.
Heh, I think there are some similarities between this series and Fullmetal Alchemist. Both leads are short and ridiculed for being short and also they have a disfigurement. Edward has his automail arm and leg. D.Gray-Man's Allen Walker has his entire left arm as well as his left eye "cursed" to become an anti-Akuma weapon. But... when I mentioned that to one of my friends, he got really pissed at me and he said he was insulted that I would even compare the two and that D.Gray-man was better than FmA in everyway possible. :rolleyes:
LOL. Well, it's a good series nonetheless. A relatively new manga. There isn't any news of it being adapted into an anime anytime soon though... I have high hopes that this series will kill FmA. That's what I believe. :D