New York City policeman Ed Lacy (Don Murray) practices his Knights of Columbus speech as he shaves and gets into uniform in the precinct locker room. On patrol in their squad car, Ed, and his partner, Billings (Treat Williams), arrest a prostitute and her African-American pimp, and threaten several pedestrians who object.
The next day, Ed leads a local rally for a conservative mayoral candidate, Kevin "Enough Is Enough" Reilly (George S. Irving), an Irish Catholic who, like Ed, stresses his ethnic and religious roots.
Meanwhile, at a local theater, Rabbit Shazam (James Earl Jones) arrives with the opening-night crowd, gets his pre-paid ticket held under the name "Ambassador Shazam" at the box office, and takes his seat. A large, African-American man wearing an identifiable Muslim caftan, Rabbit looks out of place among the middle-class New Yorkers. His expressions show contempt for the avant-garde program featuring African, classical and rock music conducted by Sally Deveraux.
Meanwhile, Ed and Billings stop at a market for snacks, and when an inebriated young African-American insults them, Ed gets angry and threatens to blow the young man's head off.
Later, Rabbit grabs Sally (Diahan Williams) outside her basement apartment, puts a knife to her throat and forces her to let him in. Sally's neighbor, Mrs. Edna Broderick, sees them in the hallway but is not sure if Sally is in danger. Inside Sally's apartment, Rabbit ties her up, searches for valuables and smashes her furniture and valuable cello; all the while he recites poetry, mocks her middle-class pretensions, and speaks with a funny accent into her tape recorder. Forcing Sally to call her parents, Rabbit tells her father that if he wants to save Sally he must send a $10,000 money order to a Western Union office on Broadway. Next door, Edna finally telephones the police. Ed and Billings get the call.
At Edna's apartment, Billings uses her phone to call for backup. Ed steps into the hall and sees Rabbit and Sally coming out of her apartment. When Ed pulls his gun, Rabbit presses his knife against Sally's throat and tells Ed to let them through, but Ed holds his ground. Hearing sirens outside, Rabbit drops his knife and turns to the wall, hands and legs outstretched. Ed kicks Rabbit's legs, and when Rabbit turns around, Ed shoots him twice in the chest. Sally screams as Rabbit's body falls on top of her. When Detective Buckley arrives, Ed tells him that Rabbit came at him with the knife.
In the ensuing investigation, Assistant District Attorney Winston (Dick A. Williams), an African-American, asks Edna why she was suspicious of Rabbit and tries to get her to admit that she called because he was black. At the precinct headquarters, Buckley and his partner, Detective Baker, ask Sally if she knew Rabbit well, and if not, why did she seem so concerned about him? When Sally has a moment alone with Ed, she thanks him for saving her but asks if he had to shoot Rabbit. He replies that Rabbit might have killed her, and killed him too. Winston interrogates Sally, but she says she cannot remember what happened because she was on the floor and did not see the shooting. Afterward, as Sally leaves the station, Ed gives her a ride home and pleads his case. He tells her about a policeman who was recently stabbed to death, and says it is good that Rabbit is dead, because otherwise the city would have to pay for an expensive trial and Sally would be dragged into court and picked apart by a defense attorney. She replies that Rabbit did not rape her, so why would a lawyer probe her sexual history? Alone in her apartment, Sally picks up a crumpled Polaroid photograph of Rabbit's body that one of the police technicians dropped on the floor. Playing a tape of a Strauss waltz, she hears Rabbit cutting in with nonsense patter. Later, she goes to the morgue and views Rabbit's body.
Meanwhile, Kevin Reilly, who had earlier seemed dismissive of Ed, sees the "Hero Cop" headline in the newspaper and decides to use Ed in his law-and-order campaign. He tells Ed he wants him on the platform at every speech and promises that he will put Ed in a "key position" in city government. When Sally sees Ed touted as a hero and possible political appointee on television, she goes to the station and tells Baker and Buckley that she wants to change her story. She admits that Rabbit dropped his knife before Ed shot him.
Later, Ed celebrates with his friends at a bar when Billings arrives to tell him that Sally has accused him of murder. Ed attends Sally's next performance and, afterward, takes her to a nearby bar in an attempt to convince her to change her mind. He tells her that he has eighteen years on the force and six commendations. A waitress, Annie, tells Sally that Ed is a hero at the bar because five years earlier he stopped a man from stabbing a waitress and, after being cut himself, shot the man to death.
Sally goes to the D.A.'s office the next day and tells Winston that Ed is a psychopath. Buckley and Baker walk her through the crime scene again. The coroner shows the two detectives Rabbit's X-rays, which show that the back of his legs suffered trauma. Seeing a new, incriminating headline about Ed in the morning paper, Reilly tells his campaign chief to "cut him loose," and when Ed shows up at a television studio later that day, Reilly explains that he will use him later in the campaign, but not now. Angry, Ed threatens Sally at the grade school where she teaches, which prompts her to again complain to Winston. Captain Stark, Ed's commander, tells him to take a vacation, and when Ed protests, Stark puts him on temporary restriction.
Though Ed takes his wife and daughter to the sea shore, he continues to stalk Sally from there. He hires a thug, Arco, to menace her, but Arco accidentally kills two pedestrians when he tries to run her down. Ed invites Arco to hide out at an abandoned farm outside of the city, and then kills him. He goes to Sally's school, knocks her unconscious, carries her to the farm, and ties her to her cello so that she cannot run. The moment Sally sees that Ed is distracted, she unties the ropes and rams the pointed stand on the bottom of her cello into Ed's chest. He chases Sally, shooting at her, but as he loses blood he becomes woozy. In a stable, she stumbles into Ed pointing a gun at her, but realizes that Ed has died from his wound.