Stu Shepard (COLIN FARRELL) is a hustling New York publicist whose life is built around lies and conducting business over the phone. That includes stopping in an out of the way phone booth to call his girlfriend, Pamela McFadden (KATIE HOLMES), so that his wife, Kelly (RADHA MITCHELL), won't see the calls on his cell phone bill.
He makes the mistake, however, of answering a call in the booth from a stranger (voice of KIEFER SUTHERLAND) who seems to know a great deal about Stu and his way of life. He also turns out to be a sniper with a rifle who warns and proves that he'll shoot him if he hangs up. Stu tries to talk his way out of the situation, but the caller is too savvy for that and seems to relish making Stu squirm. He also wants Stu to call Pam and Kelly and tell them the truth.
Things get more complicated when the sniper shoots and kills a pimp who was harassing Stu about using his hookers' phone. That ends up drawing the police and Capt. Ramey (FOREST WHITAKER) who tries to figure out what's going on.
With the caller not allowing Stu to state what's occurring, the situation becomes more tense, especially when the sniper threatens to shoot various people including Kelly and then Pam who show up after seeing Stu on the news. From that point on, Stu must decide what to do and how to deal with the caller, all while Ramey tries to resolve the situation.