While in post-production on a low-budget slasher film, Philadelphia sound technician Jack Terry (Travolta) is told by his producer that he needs a more realistic-sounding scream and better wind effects. While recording potential sound effects at a local park, he sees a car careen off the road and plunge into a nearby creek. The male driver is killed, but Jack manages to rescue a young woman, Sally (Allen), and accompanies her to a hospital. There, a detective interviews Jack about the accident, and Jack asks Sally out for a drink. He learns that the driver of the car was Governor George McRyan, and that Sally was his escort. Associates of McRyan attempt to conceal Sally’s involvement, and persuade Jack to smuggle her out of the hospital.
Jack listens to the audio tape he recorded of the accident, wherein he distinctly hears a gunshot just before the tire blow-out that caused the accident. He learns from a news report that, seemingly by coincidence, Manny Karp (Franz) was also in the park that night and filmed the accident with a motion picture camera. When Karp sells stills from his film to a local tabloid, Jack splices them together into a crude movie and syncs them with the audio he recorded, becoming suspicious that the accident was actually an assassination.
Unbeknownst to Jack, Sally and Karp were both co-conspirators in a larger plot against McRyan, a presidential hopeful. A rival candidate had hired Burke (Lithgow) to hook McRyan with a prostitute, take their pictures, and publish them so that McRyan would drop out of the race. However, Burke decided to alter the plan by blowing out the tire of McRyan’s car with a gunshot, thereby causing an accident. When the authorities arrived to find McRyan with Sally, Karp would be there to film it all. Although Burke had not planned for McRyan to be killed, he is little bothered by the development since that still accomplished the goal of eliminating McRyan.
Aware that Jack and Sally are trying to prove that the car’s tire was shot, Burke plots to destroy Jack’s evidence and kill Sally. He begins murdering local women bearing a resemblance to Sally, whose deaths are attributed to a serial killer, “the Liberty Bell Strangler”, so that he can cover-up the future murder of Sally. Jack draws Sally into his own private investigation of the incident. Though initially reluctant, she eventually agrees to cooperate with him. When they go out for a drink, Jack reveals how he left his prior career in the police force after a wiretap operation he was involved in led to the death of an undercover cop.
To help Jack investigate McRyan’s murder, Sally steals Karp’s film, which, when synced to Jack’s audio, clearly reveals the gunshot that precipitated the blow-out. Nevertheless, nobody believes Jack’s story and every move he makes is immediately silenced by a seemingly widespread conspiracy. A local talk-show host, Frank Donahue (Curt May), asks to interview Jack on air and release his tapes, to which Jack eventually agrees. Burke follows the development through a tap on Jack’s phone, calls Sally as Donahue, and asks her to meet him at a train station with the tapes. When Sally tells Jack about Donahue’s call, he becomes suspicious. He copies the audio tapes, but does not have time to copy the film before Sally’s meeting.
Shadowing a wired Sally from a distance, Jack is alarmed to see that his supposed contact is actually Burke. Immediately realizing that she is in danger, Jack attempts to warn her, but Sally and Burke slip out of range and into a parade. Jack makes a mad dash across Philadelphia, attempting to head them off and rescue Sally, but crashes his Jeep and is knocked out. By the time he awakens, Burke has gotten the film from Sally and thrown it into a river. He then takes Sally to a rooftop and attacks her. Still listening in on his earpiece, Jack spots them. Jack takes Burke by surprise and manages to stab him to death with his own weapon, but it is too late: he has already strangled Sally. A devastated Jack takes her lifeless body in his arms.
Burke’s death, combined with the loss of the film, ties up the last loose end. Jack’s audio tapes alone are insufficient to prove a gunshot and the cover-up is a success. Jack begins listening to the recording of Sally’s voice over and over again, becoming obsessed with it. In the last scene, he is back in the editing room and has used Sally’s death scream in the exploitation film. The producer is ecstatic that he found a perfect scream and plays it multiple times, forcing Jack to cover his ears.