The Best Fiction always has a bit of Real Life mixed in.

Rating: 10/10 (13 February 2016)

This excellent Scorcese combination of "Black Mass" and "Infernal Affairs" combines the semi-true story of Gangster Frank Costello (Jack Nicholson, based partly on James "Whitey" Bulger) and the story of two moles planted in the Boston Police Department, each looking for the other, and their identities hidden from each other until the very end of the story. The Context is basically recapped in the 2015 film "Black Mass" with Johnny Depp", minus the elements from "Infernal Affairs" but still retaining the idea of an FBI agent protecting his informant- In whatever underhanded ways he could.
Matt Damon as Colin Sullivan seems to be the clean cut Boston Detective, and Leo DiCaprio as "Billy" as an undercover cop who ironically becomes the most trusted member of Frank Costello's gang, and seems to be the worse off of the two. They both connect to a Police Psychiatrist Madolyn (Vera Farmiga in a more sexy role than she had in the remake of Manchurian Candidate) who becomes a pivot piece for the plot.
There is some great Age Regression for Jack Nicholson's Frank Costello, who takes Sullivan in as a young man and grooms him to become his mole. Meanwhile, "Billy" as part of his cover is trained by spending time in the Hoosegow.
It is known to Costello that there are police informants peppered through his organization, and most of the story is Costello trying to find out who these are, while Billy at the same time tries to protect who he is. Costello charges Sullivan to discover the informants, so basically the who things balance each other out. But the ways these two plot points interact, make this an interesting watch. And of course there are many examples of Scorsese humor dotted throughout in bright red grisliness- Some of these hold up to the burying alive of Billy Batts from Goodfellas.
The question is, who's the rat? And who's the real rat? When is a rat not a rat? Sometimes a rat can be two different things.