The Departed
At a young age, Colin Sullivan (Damon) is introduced to organized crime through perverse Irish mobster Frank Costello (Nicholson) who grooms him along with other youths in the Irish neighborhood of Southie (South Boston). Turning into an ambiguous father figure, Costello eventually trains him to become his mole inside the Massachusetts State Police. Performing exceptionally, all the while helping Costello remove his underworld enemies, Sullivan is accepted into the Special Investigation Unit, focusing on organized crime.
Before he graduates, the other main character, Trainee William Costigan Jr (DiCaprio) is asked by Captain Queenan (Sheen) and the ruffian Staff Sergeant Dignam (Wahlberg) to become an undercover agent, as his childhood biography as a wanderer between two distinct Boston neigborhoods and his family ties to organized crime make him a perfect infiltrator. He agrees and does time in prison on a phony assault charge to up his credibility as a criminal.
As both men infiltrate their organizations, Sullivan begins a romance with psychiatrist Madolyn Madden (Farmiga), whose profession he ridicules. Costigan sees Madden as part of his probation, developing a more respectful relationship with her.
After Costello barely escapes a sting operation, both moles become aware of the other's existence. Sullivan is assigned to find the "rat" in SIU and asks Costello for his crew's personal information to allow him to infer who is the rat within Costello's crew. Costigan follows Costello into a porn movie theater where Costello gives Sullivan an envelope with the details of his crew. Costigan then stalks Sullivan in a chase through the dark alleys of Boston during which Sullivan stabs an innocent bystander. When it is all over, both men still do not know each other's true identity.
Trying to get rid of two enemies at once, Sullivan has Queenan tailed as he meets Costigan in an abandoned building. Costello sends his men in and Queenan distracts them to let Costigan escape. For this he pays with his life as he is thrown from the roof and dies at the feet of Costigan. When the mobsters escape, Costigan pretends he has come to rejoin them. The trailing state troopers open fire on Costello's crew, which causes casualties on both sides. Later at one of Costello's safe houses, Delahunt (one of Costello's enforcers) reveals to Costigan that he is aware of his true identity just before he dies from his gunshot wounds.
His actions now under scrutiny, Sullivan is attacked by a suspicious Dignam, who is subsequently placed on paid time off. Using Queenan's bloodstained phone, Sullivan reaches Costigan who refuses to abort his mission. Sullivan learns of Costello's FBI-informant role from Queenan's diary, causing him to worry about his double-identity becoming revealed.
With Costigan's help, Costello is tracked to a cocaine drop-off, where he and his crew become trapped in a gunfight with police, resulting in most mobsters being killed. As the wounded Costello attempts an escape he is confronted by Sullivan. Costello admits he is an occasional FBI mole and tries to shoot Sullivan. Sullivan preempts him and kills his father figure.
With Costello dead, Sullivan is applauded for his actions at SIU. In good faith, Costigan comes to him for restoration of his true identity and payment, but notices the envelope containing the details of Costello's men on Sullivan's desk and flees from the building. Knowing he has been found out, Sullivan erases all records of Costigan as a trooper from the police agency's computer system.
Costigan leaves an envelope in the care of Madden, with whom he has had a love affair but who now lives with Sullivan. Some time later, Costigan sends an audio CD to Sullivan with a note to contact him on it. However, Madden listens to the CD first, hearing Sullivan and Costello's taped conversations. Sullivan walks in on her and tries to assuage her suspicions.
He contacts Costigan, who reveals that he is in possession of recordings by Costello that would implicate Sullivan as a rat in the police department. They agree to meet at the empty building where Queenan died. Costigan surprises Sullivan holding him at gunpoint, intent on arresting him. Officer Brown, both Sullivan's colleague and an academy friend of Costigan, appears and draws his gun on Costigan. Using Sullivan as a shield, Costigan gets into the elevator. As it reaches the ground floor Costigan is shot by Sullivan's other colleague Barrigan and dies. Brown appears on the scene, but is shot and killed by Barrigan who reveals himself to Sullivan as Costello's second mole. Not wanting any loose ends, Sullivan shoots and kills Barrigan. Back at the Police headquarters Sullivan blames all mole activity on Barrigan and has Costigan posthumously rewarded.
At Costigan's funeral, a pregnant Madden is seen crying. Some time later, in the final scene, Sullivan comes home one morning with a bag of groceries. Instead of Madden, Dignam is waiting for him with a suppressed pistol and kills him with a bullet to the head. The film ends with a lone rat crawling on the apartment's balcony railing, which frames the gold dome of the Massachusetts State House in the background.
