Two just-graduated officers from Massachusetts State Police Academy follow opposite sides of the law: Billy Costigan is assigned to work undercover with the Irish mobster Frank Costello to get evidences to arrest him. His true identity is only known by his superiors Dignam and Oliver Queenan. The protégée of Costello, Colin Sullivan, is promoted in the Massachusetts State Police and is the informer of Costello. Each police officer gives his best effort trying to disclose the identity of the other "rat".

Martin Scorsese wanted to shoot the film in Boston, where the story is set. But due to concerns on setting up production and politics, the producers chose New York City to double for Boston because of the state's 15% tax credit. The bulk of the movie was shot in New York City while a six week shooting schedule was split in two for Boston, shooting the first half in June and the second half in August. After the success of this film, Massachusetts created a 25% tax credit for filmmaking.

Dignam:
This is unbelievable. Who put the fuckin' cameras in this place?


Police Camera Tech:
Who the fuck are you?


Dignam:
I'm the guy who does his job. You must be the other guy.

Continuity: When Billy visits Madolyn active window of Microsoft Anti Spyware on her monitor disappears and appears in different shots.

Written by Mick Jagger and Keith Richards
Performed by The Rolling Stones
By Arrangement with ABKCO Music & Records, Inc.