The body of a young woman is found by a boy diving from a pier. The coroner finds that she died, not by drowning, but due to arsenic poisoning. The main clue in the detectives' investigation is a small heart-shaped tattoo on her hand with the letters "MAC" inside of it. Detective Steve Carrella takes his wife Teddy along with him to a tattoo parlor as he questions the owner. Later, Teddy decides to get a tattoo herself. In doing so, however, she has put herself in grave danger as the killer has come to the same tattoo parlor with his intended next victim.
A woman enters the 87th precinct headquarters and holds Detectives Kling and Meyer at gunpoint, announcing that she plans to kill Lt. Carrella as soon as he arrives. She blames Carrella for the death of her husband, who just died in prison. Carrella was the arresting officer. She is armed not only with a gun but a bottle of liquid which she claims is nitroglycerin. Soon both Detective Havilland and Teddy arrive there as well, along with a female suspect Havilland has brought in to book, and they are also held hostage as they tensely wait for Carrella to arrive.
The detectives investigate a recent series of burglaries. The crimes have identical MOs to those of several known burglars. However, when these suspects are checked out, it turns out one was dead when the crimes occurred, one was in prison, and another has an alibi that checks out completely. It appears that a copycat is at work, and the detectives come to suspect that a respected retired cop may be the one they're looking for.
Detective Bert Kling is deeply troubled after he is forced to shoot and kill an 18-year-old who was firing at him after attempting to rob a theater box office. Everyone who knew the youth describes him as a good kid. Now Kling and the other detectives try to discover the identity of the boy's accomplice who escaped.
A Hungarian immigrant locksmith realizes that two thugs used the key he made for them to enter their victim's home and beat him. He goes to the police to tell them, but his pregnant wife, fearful for his safety, begs him not to identify the men in a lineup. When another two men are killed the locksmith realizes he has to cooperate with the police after all, but things are not so simple as the killers are now free and the detectives don't know their whereabouts.
A blackmailer has been murdered, and the detectives focus their attention on three people whom he was known to have been blackmailing. Havilland also looks into a woman friend of the murder victim. Another friend of the victim seems overly interested in the police investigation.
Detective Meyer feels responsible after a mistake in judgment enables a small-time hood to escape from his custody, particularly after the man is bitten during a holdup by a watchdog---who then is discovered to be infected with rabies.
A man causes a young woman, Claudia Davis, to fall into a lake by shooting holes into the canoe she was in. Her friend Josie Thompson tries to save her but is too late, and Claudia drowns. Afterward Josie decides to dye her hair and impersonate her dead friend as Claudia had much more money and Josie, who was poor, would like to live the high life. Josie is unaware that Claudia's death was not an accidental drowning and that the killer and the man he was working for still want Claudia dead.
A car salesman is found dead in his car, shot with his own gun. The coroner believes it's a clear case of suicide, but the man's wife insists he would not have taken his own life. Lt. Carella searches for the man's last customer. His main clue is two strands of picture wire he finds tied around plumbing in locations where the customer and the victim were supposed to have met.
The detectives investigate a rash of bomb threats against businesses. Then a night watchman is found murdered. The watchman was part of a gang that is planning a bank robbery. The mastermind of the gang, Sordo, has made the threats against other businesses in order to distract the police from his real plan, and killed the watchman because he was seeing a woman and fears he might have leaked the plan to her. When Kling finds the woman at the house of the murdered man, Sordo arrives and shoots Kling and takes the woman hostage.
On the eve of his execution, a mobster vows to the press that three men who testified against him will be dead not long after he is. Very soon two of the men are killed. The third returns to the 87th precinct, where an attempt is made on his life as well. Carella decides to make it appear that the attempt was successful in order to smoke out the perpetrators.