In New York City, a waitress on her way home is assaulted by two muggers, Mugger 2 and Mugger 1, who then drive her toward a police officer for help, only for the officer to brutally break her neck. Over the next two nights, this Maniac Cop racks up more victims, and Lieutenant McCrae, under orders to downplay eyewitness reports that the killer wears a police uniform, leaks information to a journalist in an attempt to protect civilians. The city descends into panic and mistrust, with innocent patrolmen becoming targets as terrified civilians shoot at them or steer clear of the streets for fear of being mistaken for the killer.
Ellen Forrest suspects her husband Jack may be the Maniac Cop and follows him to a motel, where she discovers him in bed with [Theresa Mallory] (an officer in disguise). Distraught, Ellen is slain by the killer. Jack is arrested for murder, but McCrae believes he has been framed. McCrae pushes Jack to reveal details about his relationship with Mallory, who is attacked by the Maniac Cop while working undercover as a prostitute. Mallory and McCrae manage to fight off the killer, who feels chillingly cold through his gloves and seems to have no breath; even after they shoot him several times, he remains unfazed.
Mallory hides out in McCrae’s apartment while he pursues Sally Noland, the only person Mallory told about her affair. McCrae traces Noland to a warehouse, where she meets with the Maniac Cop and refers to him as “Matt.” Back at the police HQ, McCrae uncovers files on Matthew Cordell, an officer who was unjustly framed for police brutality and sent to Sing Sing, where he was mutilated and killed by fellow inmates after narrowing in on corruption in city hall.
When McCrae and Mallory visit Jack, they conclude Cordell is the real killer and plan to consult Sing Sing’s medical examiner. McCrae leaves to fetch him and is ambushed by Sally, who fears Cordell will turn on her. In the ensuing chaos, an officer is found dead by hanging, and Sally is beaten to death by Cordell. Hearing the commotion, Jack and Mallory rush back to the interrogation room to find corpses strewn through the halls. Mallory heads to McCrae’s car while Jack searches for Cordell, who seemingly evaporates after throwing McCrae out of a window, killing him in the process. Jack, now a suspect to responding officers, flees with Mallory.
The pair track Cordell to Sing Sing’s medical examiner, who reveals Cordell showed faint signs of life during an autopsy and was secretly released into Sally’s care, convincing him that Cordell was brain-dead. During the 50th annual St. Patrick’s Day Parade, the mayor basks in certainty that Cordell is dead as Cordell silently closes in. Mallory warns Commissioner Pike and Captain Ripley, but they refuse to believe her and have her arrested. Cordell then appears and fatally stabs Pike and Ripley, turning his attention to Mallory as she fights for her life against the killer’s blade. Mallory escapes through a window, while Jack is arrested and placed in a van that Cordell hijacks.
Mallory and another officer pursue the hijacked van to Cordell’s warehouse hideout. Cordell assaults Mallory and Jack, kills the accompanying officer, and attempts to flee in the van as backup closes in. Jack clings to the side of the vehicle and fights for control, forcing Cordell to crash into a suspended pipe that impales him. The van veers into the river and sinks, and when the wreck is pulled from the water, Cordell’s hand emerges from the surface.
An extended version adds one final twist: the mayor, confident Cordell is dead, relaxes in his office, only to be slain by Cordell as revenge for being framed.
