The Departed

The Departed is a 2006 American crime drama film, directed by Martin Scorsese and written by William Monahan. It is a remake of the 2002 Hong Kong film Infernal Affairs. The film stars Leonardo DiCaprio, Matt Damon, Jack Nicholson, and Mark Wahlberg, with Martin Sheen, Ray Winstone, Vera Farmiga, Anthony Anderson, and Alec Baldwin in supporting roles.

As a child, Colin Sullivan Damon had been introduced to organized crime by IrishAmerican mobster Frank Costello Nicholson in the Irish neighborhood of South Boston. Over the years, Costello grooms him to become a mole inside the Massachusetts State Police, until Sullivan is accepted into the Special Investigations Unit, which focuses on organized crime.Before graduating from the police academy, Billy Costigan DiCaprio is recruited by Captain Queenan Sheen and Staff Sergeant Dignam Wahlberg to go undercover, as his family ties to organized crime make him a perfect infiltrator. He drops out of the academy and does time in prison on a fake assault charge to increase his credibility. ........

Source: Wikipedia