D. Gray Man focuses on the Allen Walker who is a young Exorcist in 19th century Europe, little walker was abandoned at birth by his true parents due to his deformed hand, and adopted by a man named Mana Walker. Mourning the death of Mana, Allen accepts the Earl's offer young Walker tride to bring back to him life inadvertently his father turned into an Akuma. Suddenly Allen's left hand awakens and kills the mechanical skeleton trapping his father's soul, but not before being cursed by his father.
Young Walker's left arm can transform into a monstrous claw(anti-akuma weapon) and destroy Akuma,he also has a cursed eye that can see the suffering of the akumas, evolving machines created by the Millennium Earl to help him destroy humanity.The forces of darkness represented by the Earl, and his family the "Noah". The "good side" uses a strange holy substance called "Innocence" to give them magical weapons to fight with. Allen is a member of The Black Order; a secretive organization whose members are known as Exorcists.As ordered by his master General Cross Marian, Allen join this organaization where he meet people like him.The battle with the Millenium Earl and the Noah Family vs. the Exorcists intensifies when some of the best Exorcists begin to get killed by hoards of Akuma.He and his interactions with the Black Order and the Millennium Earl, Noahs, and the Akumas. The Black Order uses innocence(Some innocence are internal and the others are external) to combat against Akumas of the Millennium Earl's army. One Innocence, referred to as the "Great Heart" will assure orders victory. However, the Earl is also looking for the Innocence and the Heart with the intent to destroy them.
In the mean time Allen learns he is the prophesied "Destroyer of Time" who is the only one who can defeat the Millennium Earl. As the series progresses, Allen meets many of the members of the Black Order headquarters as well as those in other branches worldwide. The Order suspect that Allen might betray them, and he is eventually confined. The Noah frees him to rescue Mr. Walker from the Apocryphos, a sentient Innocence that guards the Heart, and the stories go on....