Two rats. One mob boss. Leonardo DiCaprio plays Billy, a trainee cop who has a bad family history. He is sent by Captain Queenan, played by Martin Sheen and Sergeant Dignam, played by the excellent Mark Wahlberg, to infiltrate the Boston mob, run by Jack Nicholsons Frank Costello. Matt Damon plays Colin, a policeman who rises fast through his ranks, but is secretly working for Costello. Both characters bounce off eachother, while never actually meeting, while each tries to find the others identity, while coming unknowingly close (both are interested in the same woman). That is the basic set up for this brilliantly tense mob thriller, directed by Martin Scorcese.