In New York City, a man, face unseen, puts on a police officer’s full dress uniform with a name badge reading “Cordell.” As Cassie Philips walks home after her shift at a bar, two thugs attack her. She escapes and they chase her into a park. Cassie sees a police officer standing in the shadows and runs to him for help, but the officer strangles her to death. The next morning, Lieutenant Frank McCrae and his partner investigate Cassie’s death. McCrae wonders if two young thugs are capable of inflicting such severe injury, or if he believes their story about a policeman killing the young woman. That evening, a couple stops at a red light and the murderous officer approaches. He motions for the driver to get out, slices the man’s throat and tosses his body onto the windshield. The woman slides to the driver’s seat and speeds away. McCrae meets with police Commissioner Pike and theorizes the killer left the woman alive because he wanted a witness. The commissioner believes the killer dresses as an officer to discredit the police, but McCrae insists on investigating fellow officers. Commissioner Pike agrees but orders McCrae to keep the inquiry quiet. That evening, the killer handcuffs a musician, but the man runs away, screaming for help. When the musician trips and falls headfirst into wet cement, the killer pushes the man’s face into the newly poured sidewalk. In the morning, police drill the dead man’s face out of the hardened cement. To warn the public, McCrae shares the story with Gina, a local newswoman, and she reports on the “maniac cop.” In an industrial area, a woman’s car breaks down as she listens to Gina’s report. When an officer approaches her vehicle, the terrified woman pulls out a gun and shoots him in the head. Later, Ellen Forrest updates her scrapbook with articles on the “maniac cop” while her husband, police officer Jack Forrest, puts on his uniform. Ellen worries that he works overtime to get away from her, and Jack complains that she has stopped attending couples therapy. As they argue, Ellen reveals she is afraid of Jack, and afraid of being alone. He is certain she is safe inside their house and notes she has a gun for protection. After Jack leaves, the telephone rings and the female caller taunts Ellen that Jack is the killer. Ellen hangs up the phone, pockets her gun and follows her husband. When Jack stops at a motel and enters a room, Ellen gets the room key from the front desk and lets herself in. She discovers Jack in bed with officer Theresa Mallory. Ellen threatens to shoot them, but leaves instead. As she rushes down the street, the maniac cop pulls her inside a van. When the maid cleans Jack’s motel room the next morning, she discovers Ellen’s dead body on the bed. Jack is arrested and during interrogation, Captain Ripley and Lt. McCrae reveal that Ellen wrote in her diary she feared Jack was the “maniac cop.” Jack does not want to ruin Theresa’s reputation and plans to give the police a week to find the real killer before naming her as his alibi. McCrae thinks Jack is being framed and worries the killer will go after Jack’s alibi. McCrae questions Jack alone, learns about Theresa and leaves to find her. Theresa works undercover for the vice unit, dressed as a prostitute. As she walks down an alley, she sees an officer in the distance and identifies herself. The killer officer grabs Theresa by the throat and tosses her aside as McCrae arrives. Both officers shoot, but the bullets do not faze the killer and he disappears. McCrae asks Theresa if anyone in the department knows about her affair with Jack, and learns that she spoke with police clerk Sally Noland. McCrae tells Theresa to hide in his apartment while he returns to the station and introduces himself to Sally, noticing she wears a leg brace and walks with a cane. McCrae informs Sally of the attack on Theresa, and follows when Sally leaves work and heads to pier fourteen, a dilapidated waterfront property slated for demolition. McCrae sneaks onto the property and watches Sally Noland meet with the killer. Sally tells Cordell not to go after Theresa because the police believe Jack is the killer, and when their guard is down, he can murder Commissioner Pike and the mayor. McCrae returns to the police station and reads newspaper clippings about Matt Cordell, a former officer once considered a hero. However, Cordell was known for “shooting first and asking questions later” and was convicted of violating people’s rights. The police archives clerk tells McCrae that politicians in City Hall murdered Cordell by placing him in Sing Sing prison with the criminals he had arrested. The archives clerk also reveals Cordell dated Sally Noland, who attempted suicide by jumping out of a window after his conviction, and was crippled. At pier fourteen, Cordell remembers being attacked in the prison shower by three convicts who stabbed him and viciously slashed his face. Theresa and McCrae visit Jack and announce that Cordell is the murderer. Jack insists Cordell died in jail, but Theresa and McCrae plan to question the prison’s medical examiner the following day. McCrae leaves Theresa alone with Jack and goes to Sally Noland’s office. Sally attacks McCrae with her cane, screaming that Cordell will kill her because she is no good to him anymore. She staggers from the room and sees a dead officer hanging in the hallway. McCrae rushes into the hallway as Cordell pulls Sally into an office and kills her. McCrae shoots Cordell, but he is unhurt and attacks the lieutenant. Jack and Theresa hear gunfire and rush into the hallway which is littered with dead police officers. Jack tells Theresa to run outside and get a police car ready to go. As Jack runs to help McCrae, Cordell tosses McCrae’s body out the window and it lands near Theresa’s police car. Officers think Jack is the killer and try to stop him, but he eludes the officers and drives away with Theresa. In the morning, Jack pretends to be McCrae as he and Theresa question Dr. Gruber, Sing Sing prison’s medical examiner. Gruber admits that Cordell did not die in the attack, but suffered such severe brain damage he could be considered legally dead, and Sally Noland convinced Gruber to sign the death certificate. Jack and Theresa return to the city as the St. Patrick’s Day parade starts. At the police station, Jack waits outside while Theresa tries to convince Commissioner Pike and Captain Ripley that Cordell is the killer. However, before his death, McCrae left a message for the commissioner that a female officer was involved, and they arrest Theresa, believing she is part of Jack’s murderous plan. The commissioner and Ripley start for the parade, but are slaughtered by Cordell. The arresting officer is also murdered, but Theresa escapes Cordell by climbing onto a window ledge. As Jack rushes to help Theresa, officers arrest him and place him in a police van. Cordell jumps in the driver’s seat and speeds off. Theresa climbs down the fire escape, gets into a police car and orders the officer to chase the van. As they speed through the city, Theresa hears on the police scanner that Cordell is headed for the waterfront and she directs her driver to pier fourteen. At the pier, Cordell speeds the van into the abandoned warehouse, grabs a fire axe and breaks the lock on the back door. Jack launches out the door at Cordell as Theresa enters, shooting. Cordell tosses Jack at her and she loses her rifle. Cordell’s scar-covered face is revealed as he grabs the rifle. Theresa’s driver rushes in with his rifle but is shot to death by Cordell. As more police arrive, Cordell tries to escape in the van. Jack jumps on the running board and fights Cordell as the van races across the pier. As Cordell punches Jack, he does not notice a harpoon hanging from rigging until it is too late. The harpoon plunges into Cordell’s chest and the van speeds off the pier. Jack falls away from the vehicle as it crashes and sinks. Later, Jack, Theresa and armed policemen surround the vehicle as it is raised. The van is empty. However, beneath the pier, Cordell’s scarred hand reaches up from the depths.