The movie is set in Boston, Massachusetts. Irish Mob boss Francis "Frank" Costello (Jack Nicholson) plants Colin Sullivan (Matt Damon) as a mole within the police force. At the same time, the police get William "Billy" Costigan (Leonardo DiCaprio) to gain access into Costello's crew. This leads to Sullivan and Costigan each trying to find the other's identity before his own cover is blown.