Q:Build a movie plot around this: Sullivan is ambushed by whom? Dignam
A:
As a child, Colin Sullivan (Damon) had been introduced to organized crime by Irish-American 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.Each man infiltrates his respective organization, and Sullivan begins a romance with police psychiatrist Madolyn Madden (Farmiga). Costigan is seeing her under the terms of his probation, and they begin a relationship, too. After Costello escapes a sting operation, each mole becomes aware of the other's existence. Sullivan is told to find the "rat" and asks Costello for information to identify the informer.Costigan follows Costello into a porn theater, where Costello gives Sullivan an envelope containing personal information on his crew members. Costigan chases Sullivan through Chinatown. When it is over, neither man knows the other's identity. Sullivan has Queenan tailed to a meeting with Costigan on the roof of a building. Queenan orders Costigan to flee while he confronts Costello's men alone. The men then throw Queenan off the building to his death. When they exit, Costigan pretends he has come to join them. Television news reveals that crew member Delahunt (Rolston) has been an undercover cop, working for the Boston Police Department. Dignam resigns rather than work with Sullivan, who he suspects is the mole after he is asked why he had Queenan followed.Using Queenan's phone, Sullivan reaches Costigan, who refuses to abort his mission. Sullivan learns from Queenan's diary of Costello's role as an informant for the...