The Police Department of Massachusetts is facing the biggest organized crime ring in the city. The strategy is to finish from within the powerful mob boss Frank Costello (Jack Nicholson ). The charge to infiltrate the band is a young rookie, Billy Costigan (Leonardo DiCaprio ). While Billy tries to gain Costello's trust, another young cop, Colin Sullivan (Matt Damon ), rises rapidly category and serves on the Special Investigations Unit, elite group whose mission also is to bring Costello down. What his superiors do not know is that Colin is working for him.