The Departed

Colin Sullivan, first introduced to Boston Irish mob boss Frank Costello in the 1980s, has been groomed for years as Costello's spy inside the Massachusetts State Police (MSP). Sullivan is assigned to the Special Investigation Unit, where Captain Queenan and Sergeant Dignam assign another police academy recruit, Billy Costigan, to infiltrate Costello's organization.
Costigan, serving a prison term as cover, draws Costello's attention by committing several crimes and is recruited into Costello's gang. Costigan's mental health declines from increasing involvement in Costello's violent criminal activities, but Queenan and Dignam convince him to remain undercover. Sullivan begins dating police psychiatrist Madolyn Madden, who becomes Costigan's court-ordered therapist.
Costigan notifies the MSP that Costello will be selling stolen microprocessors to Chinese mobsters. When Sullivan helps thwart the attempted sting operation, Costello and the MSP realize they've been compromised. Costigan and Sullivan are each tasked to find the opposing mole. Costigan and Queenan learn that Costello is a protected FBI informant. Costigan begins an affair with Madden.
Costigan follows Costello and sees him pass Sullivan information on his crew. Costigan is unable to identify Sullivan, who realizes he is being followed and mistakenly stabs a passerby before fleeing. Lying to his fellow officers to have Queenan followed, Sullivan realizes Queenan is meeting with his mole and informs Costello's gang.
Costigan, fearing discovery as the mole, meets with Queenan to abort the operation. Queenan helps Costigan escape, but Costello's men arrive and throw Queenan from the building to his death. Fatally wounded in the ensuing firefight with police, Costello's henchman Timothy Delahunt tells Costigan that he knows he's the mole before dying.
Following Queenan's murder, Dignam is suspended after confronting Sullivan about cooperating with the FBI. A news report identifies Delahunt as a Boston Police Department undercover officer, but Costello suspects it's a ruse to protect the real mole. Sullivan directs the MSP to tail Costello, resulting in a gunfight that kills most of Costello's crew. Sullivan confronts a wounded Costello, who admits to being an informant before Sullivan kills him.
His assignment finished, Costigan reveals himself to Sullivan but recognizes Costello's envelope on his desk and deduces that Sullivan is Costello's mole. Costigan flees, and Sullivan realizes he has discovered the truth. Sullivan deletes Costigan's police records. Costigan leaves an envelope of evidence with Madden, who finds a recording of Sullivan's incriminating conversations with Costello.
Meeting Sullivan on the rooftop where Queenan was killed, Costigan arrests him. Trooper Brown, Costigan's police academy classmate, arrives as Costigan holds Sullivan at gunpoint, declaring he has evidence tying Sullivan to Costello. Taking the elevator to the lobby, Costigan is shot and killed by Trooper Barrigan, who reveals he is another of Costello's spies in the MSP. Brown is shot by Barrigan, who is then shot by Sullivan, framing Barrigan as Costello's only mole.
Sullivan recommends Costigan for posthumous commendation, but after Costigan's funeral, a pregnant Madden leaves him. He arrives home to find Dignam, who shoots him in the head and departs.