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.
A boxer's punching bagged is rigged, injuring one of his assistants. The detectives try to discover who is trying to send a message to the fighter---or to his trainer. Is it a mobster who has tried to control the fighters in the past, or someone who the trainer and fighter trust?
After returning drunk to his bachelor party the night before his wedding, Cleve Tompson remembers finding his fiancee's body in her apartment. He calls police and tells them that he had to have killed her. But after checking out the evidence in her room, the detectives don't believe he could have done it. They find that the girl had recently paid large amounts of money to several other men. Meanwhile, Tompson, still believing he had to have done it, calls the newspaper and they print his confession to them on the front page.
A human hand is found in a flight bag that was thrown in a trash can. The detectives try to match it to a recent missing person. Their main focus is on a man who was reported missing by his wife after she kicked him out because she thought he was having a fling with a model. When they investigate the model, they find that she too has been reported missing.
The detectives investigate a series of phone booth bombings. One suspect, feeling persecuted about being accused due to his past record, decides to copy the bombings and take advantage of them to commit robbery as well.
Acting on an anonymous tip, the detectives pick up Greg Brovane in connection with a robbery in which two men were killed. On interrogation, Brovane names three accomplices. However, the detectives can not find any information on any of the three accomplices. What they don't know is that Brovane is a patsy who was hypnotized and subjected to intense brainwashing by the actual perpetrators, and that the three accomplices he has named do not exist.
Right after returning from her honeymoon, a policeman's wife is shot in the back and seriously wounded by a sniper. The only apparent witness is a young boy who strangely claims to have seen a suspicious person but not to have heard the shot, even though he was right across the street. The detectives focus their investigation on several men who threatened the policeman because they blamed him for their imprisonment.
Two ex-convicts kidnap a young man, believing him to be the son of shoe manufacturer Douglas King. Actually, the boy is the son of the manufacturer's chauffeur. When the kidnappers discover this, they still demand that King pay $250,000 for the boy's safe return. However, King, believing this will ruin him financially, tells the police he is not going to pay the ransom.
The detectives investigate a burglary, which escalates when the burglars later return and shoot the owner. Meanwhile, Meyer becomes concerned about his recurring chest pains. He sees his doctor at the insistence of his wife, but does not tell his fellow officers.
A young woman is found murdered in the liquor store she was working in as a clerk. Though it at first looks like a burglary, the detectives soon discount that theory and look into the girl's acquaintances, particularly as she was engaged in a custody battle with her ex-husband, and was romantically involved with her employer and hoping to marry him when he divorced his wife.
Veteran beat cop Otto Forman is plagued with guilt after his partner is shot and killed during a robbery. The detectives also have questions about Forman, particularly after they learn that the gun used in the robbery and murder was stolen from him five years ago, and that the likely killer is his estranged son.
In response to an incident of vandalism at a store, Kling tries to persuade baseball player Larry Brooks to start a ball club for young kids in the neighborhood he grew up in. Larry finally agrees, after learning that his parolee older brother has become involved in a numbers racket. Unfortunately, Larry's attempts to keep his brother out of further trouble just wind up digging a deeper hole for both of them.
Havilland's usually cold and cynical heart is warmed by a boy who stole a wreath for his mother's grave, so he helps out the youth by buying him a wreath. The boy, Ramon, wants to somehow repay the detective by helping him out. What's unknown to the police is that Ramon's father, along with a janitor at the precinct, is part of a group planning to assassinate the prime minister of a Latin nation whom the precinct officers have been asked to help protect. When the group draws straws, Ramon's father is the one picked to do the killing.
The detectives are on the hunt for a wounded boxer who, after being shot by the owner of the store he was trying to rob, later killed an off-duty cop by slugging him through a glass window. Working with the 87th Precinct detectives is an officer from a much less crime-ridden precinct. As they are about to arrest the boxer, the new officer makes an unwise move which in the process almost gets himself and Carella killed.
Veteran officer Mike Power, only months away from retirement, is transferred to the 87th from Central Station after he is shot and a prisoner he was assigned to protect killed. While working at his new precinct Power starts giving sensational scoops to the press about the police cases he is investigating, something which does not please the other officers - and could put him in danger as well.
After a public stenographer takes the dictation of a millionaire's last will and testament, three apparent attempts are made her life, and she is approached by a man offering $100,000 if she claims the millionaire was not of sound mind at the time the will was dictated. Havilland develops a close relation with the girl while trying to protect her.