Matt Damon and Leonardo DiCaprio are supremely cast as mirror images of each other. As two cops, they see themselves in each other, traveling in the world of an Irish-American gangster Frank Costello, (Jack Nicholson) and both eventually becoming informants: one for Costello, one against him.

Matt Damon is Colin Sullivan, a mafia man covertly working as a cop who has been groomed by Costello since he was a child, and when he joins the Massachusetts State Police he is in a position to tip off the crime boss about upcoming raids.

Meanwhile Billy Costigan (DiCaprio), a disgraced ex-officer with anger-management problems, has made his way into Costello’s inner circle. But, he is actually still working for the force undercover, at the arrangement of Captain Queenan (Martin Sheen) and his high-wired partner (Mark Wahlberg).