Frank: I got this rat... This...gnawing, cheese-eating, fucking rat... And it brings up questions. You know, say, Bill, like, you're the new guy. Girlfriend, mm-hm. Now, why don't you stay in the bar that night I got your numbers? Social-security numbers.
Costigan: Yeah.
Frank: Everybody's fucking numbers. Costigan: Is...? Is there something that you just want to go ahead and ask me, 'cause I'll give you the fucking answer, all right? Frank, look at me. Look at me. I'm not the fucking rat. Okay? I'm not the fucking rat.

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Frank is confronting Costigan about the possibility of Costigan being the rat in his crew.

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