"The Departed" follows two Massachusetts moles working opposite sides of the same organized-crime investigation, and both men die before the truth fully surfaces. Billy Costigan goes undercover inside Irish mob boss Frank Costello's crew for the state police, while Colin Sullivan secretly feeds Costello information from inside that same police department, and the ending turns on which man's double life collapses first.
The film, directed by Martin Scorsese from William Monahan's script, is a Boston-set remake of the 2002 Hong Kong thriller *Infernal Affairs*. Released by Warner Bros. on October 6, 2006, it grossed roughly $290 million worldwide against a $90 million budget and won four Academy Awards, including Best Picture, at the February 25, 2007 ceremony, according to Wikipedia's production and reception summary.
Table of Contents
- Who Is Working for Whom?
- How Does Costello Go Down, and Who Kills Him?
- What Happens on the Rooftop, and Who Kills Costigan?
- How Is Sullivan Finally Exposed, and How Does the Film End?
- What Does the Final Rat Shot Mean?
- Frequently Asked Questions
Who Is Working for Whom?
The plot rests on a mirror-image setup. Billy Costigan (Leonardo DiCaprio) is an undercover Massachusetts State Police trooper who infiltrates Frank Costello's (Jack Nicholson) Irish mob crew, while Colin Sullivan (Matt Damon) is a State Police officer who is secretly Costello's informant inside the department, as laid out in Wikipedia's plot summary. Neither man knows the other's identity at first, but each is tasked with the same job from opposite sides: find the rat.
That symmetry is the engine of the film. Costigan reports to his police handlers while risking exposure inside Costello's operation, and Sullivan reports to Costello while climbing the ranks of the same unit hunting him. The two men are investigating each other without realizing it, which sets up the collapse that follows once both identities start to crack.
How Does Costello Go Down, and Who Kills Him?
Sullivan engineers a drug-drop raid with information Costigan has fed back to the police, and the operation wipes out Costello's crew in a shootout, according to the Wikipedia and TV Tropes account of the raid. In the aftermath, Costello reveals to Sullivan that he, too, has been an FBI informant all along, and Sullivan shoots and kills him to keep that information buried.
That moment removes the mob boss from the story but does not resolve either mole's exposure. Costigan has lost his cover target, and Sullivan has murdered the one man who could prove Sullivan was ever connected to Costello's organization. Both men are now more exposed than before, not less.
What Happens on the Rooftop, and Who Kills Costigan?
Once Costigan and Sullivan each discover the other's true identity, Costigan corners Sullivan at gunpoint on a Boston rooftop, demanding the records that would clear his own name and expose Sullivan as Costello's inside man, as described in Looper's ending breakdown. It looks, briefly, like Costigan has won the confrontation. That resolution doesn't hold.
Costigan is shot dead in a police-headquarters elevator by Trooper James Barrigan, who turns out to be a second mole working for Costello that neither the audience nor the other characters knew about, per the same Looper account. Sullivan immediately shoots Barrigan dead at the scene and fabricates a cover story, letting himself be publicly credited with taking down Costello's last mole, according to CinemaBlend's ending explainer. The twist means the one man who could have exposed both infiltrations is dead, and Sullivan walks away a decorated officer.
How Is Sullivan Finally Exposed, and How Does the Film End?
Sullivan's cover holds only until Madolyn Madden (Vera Farmiga), a psychiatrist who had relationships with both Costigan and Sullivan and later married Sullivan, hands over records and evidence from Costigan that tie Sullivan directly to Costello, per Looper's reporting. She gives this material to Sean Dignam (Mark Wahlberg), a police captain who worked closely with Costigan's handlers.
In the film's final scene, Dignam does not arrest Sullivan. He ambushes Sullivan inside his own apartment when Sullivan returns home and shoots him dead on the spot, acting outside the law to avenge Costigan and Captain Queenan, according to the same Looper breakdown. There is no trial and no public reckoning — Dignam settles the score himself.
What Does the Final Rat Shot Mean?
The closing shot pans from Sullivan's corpse to a rat crawling along his balcony railing, with the Massachusetts State House's golden dome visible in the background. Critics have read the image as a symbol of institutional corruption that persists even after both moles are dead, as described in Slashfilm's analysis of the ending.
Scorsese himself has said the shot is "not meant to be literal." He has described it instead as a comment on the film's broader themes of paranoia, betrayal, and the uncertainty of knowing whom to trust, a remark quoted via CultureSnob. The rat isn't a stand-in for a specific remaining informant — it's the film's last word on a world where the line between cop and criminal never fully resolves.
Frequently Asked Questions
Did Colin Sullivan get away with being Costello's mole?
No. Madolyn Madden gave Sean Dignam records tying Sullivan to Costello, and Dignam shot Sullivan dead in his own apartment rather than arresting him, per Looper.
Who killed Billy Costigan?
Trooper James Barrigan, a previously unknown second mole working for Costello, shot Costigan in a police-headquarters elevator, according to Looper.
Why does Sullivan kill Frank Costello?
Costello admits to Sullivan that he has also been an FBI informant, and Sullivan kills him to prevent that fact — and his own ties to Costello — from surfacing, per Wikipedia and TV Tropes.