A continuation of 2006’s D.Gray-man, this series picks up an indeterminable amount of time after the point that one ended. The protagonist(s) get their first chance to visit the Black Order’s new headquarters since being destroyed in the previous series, and Allen finds himself treated like an enemy after learning that he may no longer be alone in his own head.
The show does a decent job explaining the plot for people who may not have seen the earlier 103-episode series, but doesn’t elaborate on or explain the characters’ relationships with one another. So while you can jump right into the story here, treating it as an example of in medias res, you’ll probably not get much out of it since the character behavior/interactions are fairly central.
More Information:
aniDB
Fansub.co
Funimation
Wikipedia