Allen Walker is part of the Black Order an organization of exorcist that hunt tormented human souls called Akuma made by the Millennium Erul who sees to bring chaos and destruction to the world to kill humans along with his group of decedent's of Noah. They seak the Heart the source of a power called Innocents.
Innocents is a power that picks it user it is the made into a wepoen that can kill amd fight against the akuma once you gain innocents you are taken to be trained ti become and exorcists
[STORY SPOILERS] frist half
Allen comes to join the oder by his masters orders there he meets leenalee and the rest of the order members of the European branch but as he joins they think he is not human from his curse marking and his arm that holds his innocence
His master Roy Cross is missing along the scerch for him and on a new caes of innocents and a town lost in time. Allen meets a member of the Noah Family Road camalot and her strange powers she said she human but a more advanced human after a battle she leaves him and leenalee injured and near death.
That's all for the stroy I'll tell.