Major spoilers ahead, obviously, for The Departed (2006). I give some refreshers to jog people’s memory.

Costigan (DiCaprio) is an undercover cop. Toward the end, after the death of crime lord Costello (Nicholson), whose gang Costigan infiltrated, he returns to the precinct to get his identity back and get paid.

At this point, Sullivan (Damon), who is the rat that fed police information to Costello, pretends to be a clean policeman to Costigan and agrees to get him his identity back. Costigan gives Sullivan the password to his file for this purpose.

After Sullivan steps outside his office to get this done, Costigan finds evidence (the ‘Citizens’ envelope) on Sullivan’s desk that exposes Sullivan as the rat. Costigan decides to leave right then and there. Sullivan returns to his office, sees that Costigan has left, and realizes that Costigan knows that he is the rat.

I don’t understand why Costigan decides to tip his hand and not pretend to Sullivan that he doesn’t know until after he has his identity back. Leaving means: no money, no identity, less support from other officers, running the risk of having his file deleted (which Sullivan then promptly does), and with that, no ability to prove he was undercover, as well as, again, tipping off Sullivan generally, which gives Sullivan time to prepare.

Further, Costigan should have realized that, Costello being dead and his crime operation having ended, Sullivan has an incentive to let this whole thing go. That doesn’t mean Costigan should have let things go as well, just that there was no need to react as quickly as he did.

I have found two thoughts on this issue in the responses to this Reddit post.

OP asked:

Why would Costigan run away after finding the ‘Citizens’ envelope?

Somebody responded:

Costigan fled the officer because he had no idea how many dirty cops were in the department. He left to make sure that he could confront Sullivan on his terms, after he found hard proof that Sullivan was dirty.

But not getting his identity back means it’s even harder to fight dirty cops. He’d be in a better position to confront Sullivan after restoring his identity.

Somebody else responded:

Costigan is a former police officer with a felony (?) conviction who is a known associate of a recently murdered crime lord/ suspect in the killing of an MA state police commander who’s experiencing pretty severe anxiety.

As the OP points out in response, Costigan could have proved that he was legit by providing his password to other officers, so his criminal background doesn’t matter. I should add that he would have had proof of payment as well. Regarding anxiety, if this decision was simply a mistake by Costigan thinking on his feet, I think a conscientious director would have shown a scene of Costigan regretting the decision, given how important it is to how the plot develops.

Am I missing something, or is this a legitimate plot hole?

  • 5
    I gotta re-watch to be sure, but IIRC, the thinking is that if he were to sit there like a dummy, Sullivan would return and immediately arrest him as a criminal, which would eliminate all his options for proving his identity. None of the other cops on the force are going to listen to a criminal who says he can "prove he's actually a cop by giving them his password." His options after leaving are not great, but at least he has options. Once Sullivan handcuffs him, he is certain to be quietly murdered in a cell, probably that evening.
    – Tom
    Mar 1 at 5:04
  • "None of the other cops on the force are going to listen to a criminal" Deputy Brown would. And some of the cops would know how to locate Dignam. Mar 2 at 7:06

2 Answers 2


First, Billy Costigan had to have a criminal record to be able to go undercover and infiltrate Costello's crime organization:

Costigan serves a term in prison to set up his cover and further commits a series of crimes in an effort to draw the attention of Costello and his primary enforcer Arnold French, who later recruit Costigan into the organization due to their affection for his uncle. The departed - wikipedia,

After Queenan has been murdered and Dignam suspended, Costigan has only one way left to prove who he really is: his "hidden" file. Once he realizes Sullivan is working for Costello, he's trapped. No one has his back. Were you alone, convicted felon, in a police station, with a high-ranked officer that works for the other side and might be able to arrest you right after storming into the room, were you than man, would you wait for the cuffs or leave ASAP, run as far away as you can, and try to find a way out by yourself?

I think that's what Costigan thought. He knew he needed time to gather evidence of the plot, to prove himself innocent, but not the kind of time you'll have plenty of when locked in a cell.

Being left alone, no one to help him, he had to handle things by himself. 1. escape 2. find a solution. That's why, later, he caught Sullivan on the roof, 1-on-1, to try and clear his name extracting confession from him. Only then he could go back to the police with evidence to back up his claim and explain everything.

if you run away, you throw your phone, you hide, you avoid the places you usually frequent, you don't call people (but eventually one you absolutely trust)... and so on. When you discover the cop you just told everything is a f*** b***, you don't wait for him to come back and betray you, you don't try and play smart, trying to act like "you do not know and it'll be fine". You run to save your skin and don't take any chances...

  • "Billy Costigan had to have a criminal record" I was referring to his criminal activity prior to the staged prison term. Dignam mentions something about a folding chair during his interview with Costigan. "would you wait for the cuffs or leave ASAP" Couldn’t Sullivan just geolocate Costigan’s cellphone if he wanted to arrest him? Not sure running is the solution here. It just means Sullivan now definitely sees Costigan as a loose end. Mar 2 at 7:16
  • "Couldn’t Sullivan just geolocate Costigan’s cellphone": if you run away, you throw your phone, you hide, you avoid the places you usually frequent, you don't call people (but eventually one you absolutely trust)... and so on. When you discover the cop you just told everything is a f*** b***, you don't wait for him to betray you, you don't try and play smart, trying not to know. You run to save your skin and don't take any chances...
    – OldPadawan
    Mar 2 at 8:01
  • "if you run away, [...] you don't call people" Doesn’t Costigan call Sullivan to meet up? "You run to save your skin" In other contexts that would make sense, but in this particular case, I still don’t see how that doesn’t put Costigan at a major disadvantage. Mar 3 at 0:10
  • "Doesn’t Costigan call Sullivan to meet up?": does he know Sullivan is the rat when he calls? No. Would he call Sullivan if he knew? Hell no! That's basic survival toolkit: in that situation, when you discover you've been betrayed, you don't wait, you run. Staying and waiting for Sullivan is being the rabbit in the middle of the road at night, seeing the lights and waiting for the car...
    – OldPadawan
    Mar 3 at 0:52
  • "does he know Sullivan is the rat when he calls?" Yes, I’m referring to Costigan planning their meeting on the rooftop. Mar 3 at 1:36

Just an addition to OldPadawan's answer, IIRC, as i havent seen it for ages either, Costigan is in the office, and has just given the password to Sullivan.

Armed with the password and with the contents of Costigans file - and proof that Costigan is a cop - Sullivan can play this either way:

  • nice good cop, give Costigan his file and his life back (hoping Costigan never finds out he is the rat) - or
  • still play bad cop as he knows Costigan is a loose end - a potential threat to him in the future if he ever makes the connection - so delete the file and Costigan is now arrest-able with no proof

Costigan in the meantime, realizes who the rat is. He also knows the rat can play the situation two ways from now - play nice, give the file, and his life, and walk away (but, Sullivan is a rat, he is going to protect himself, right?) - or take him down, if not now, then later, but he in all likelihood would destroy any proof Costigan had that he was undercover (Sullivan is a rat, he's going to delete that file and take me out).

Costigan bets on the latter, that Sullivan would tie up a loose end, and leaves ASAP.

(If i have just reiterated what everyone else said, apologies..)

  • 1
    @DennisHackethal Arrest is probably not top concern. Sullivan could do anything to Costigan. Shoot him dead right there and plant a gun on him, or bat him over the head from behind and quietly dispose of him later. Costigan is in a very weak (and likely life-threatening) position and has to make a snap decision, he's already been let down by the police who promised to protect him, he retreats. Mar 11 at 10:56
  • 2
    He who fights and runs away lives to fight another day Mar 11 at 10:56
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    @DennisHackethal but this isn't me or you who is worried about this, this is Costigan who has been hauled over the coals, put through an emotional shredder, and now finds himself effectively trapped in a building full of people who think he's a criminal with a man who he know is capable of very bad things. He's very scared and vulnerable, I'd struggle to make any decision in his situation, i'd probably just cry. Mar 12 at 11:56
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    "I'd probably just cry.": ...or escape and run, like Costigan :)
    – OldPadawan
    Mar 12 at 15:36
  • 1
    Nah, full panic attack for me. Mar 12 at 17:48

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