Set in South Boston where the Massachusetts State Police Department is waging an all-out war to take down the city s top organized crime ring. The key is to end the reign of powerful mob boss Frank Costello (Jack Nicholson) from the inside. A young rookie, Billy Costigan (Leonardo DiCaprio), who grew up in South Boston, is assigned to infiltrate the mob run by Costello. While Billy is working to gain Costello s trust, another young cop who came up from the streets of Southie, Colin Sullivan (Matt Damon), is quickly rising through the ranks of the state police. Earning a spot in the Special Investigations Unit, Colin is among a handful of elite officers whose mission is to bring Costello down. But what his superiors don't know is that Colin is working for Costello, keeping the crime boss one step ahead of the police.