When a man convicted of armed robbery is mortally wounded in a prison knife fight, he calls on Hammer to perform a last service - retrieve the loot he hid before he was arrested and use the money to ransom his wife who has been kidnapped.
After discovering his office has been ransacked, Mike receives a threatening phone call ordering him off "the case" and to drop his client, Sharon O'Closkey. Mike refuses, even though he wasn't working on a case and didn't have a client named O'Closkey. When the mysterious client calls and invites the private investigator to her room, Mike quickly agrees, only to fall victim to drugged meal. When he awakens, he discovers he's in Las Vegas and his beautiful client claims he flew to Nevada willingly to help her find her missing father.
Mike responds to a phone call from a small-time junkie requesting protection too late; he finds his friend shot to death in a Chinatown alley. Not satisfied that the police will exert themselves to find the man's killer, Mike launches his own investigation and links the crime to an earlier unsolved murder.
Mike returns to his old neighborhood when gunsels working for a protection racket murder a boyhood friend. Before long, his investigation into the gang's criminal activities results in more fatalities.
While eating in a hotel restaurant, Mike spots a man take a package from a woman's table. Even though the package contains a necklace with a broken clasp as the man claimed, Mike smells foul play and traces the woman who left the box. When he arrives at her house, he learns that she was pushed down her stairs and badly injured. Investigating further, he discovers she is the victim of a vicious blackmailer.
Mike investigates a disheveled man's wild tale about a man that he killed six months ago dying a second time in a car accident two days ago. When his client turns up dead, Mike follows his only clue, the phone number of a pretty piano player billed as "Miss Patti".
A nightclub singer asks Mike to get her out of a contract that's held by a mobster. The man claims the rising star who she owes him $50,000 "for expenses" plus 30% of her salary in perpetuity. While Mike is working on the contract, the singer receives a blackmail threat. Mike tries to retrieve the indiscreet photograph, he finds the blackmailers dead and the negative gone.
Mike is hired to retrieve incriminating letters from a blackmailer, but soon learns that the package with $50,000 was really the ransom money to payoff kidnappers. When the woman who collected the ransom from Hammer is double-crossed, she tries to get revenge on her partner but is killed before she can meet with the private detective. Hammer investigates the dead woman's friends and co-workers to mete out his kind of justice upon the kidnapping murderer.
The lawyer for a wealthy couple asks Mike to help their niece kick her heroine habit. Mike works on two fronts: helping the young woman withdraw from the drug and finding the pushers that got her hooked in the first place.
Mike is asked to investigate help a young man who has been arrested for murder in a small Pennsylvania town. He thinks the case against his client is pretty good, but when the local district attorney and deputy sheriff try to run him out of town, he decides to investigate further.
While waiting for his car to be repaired at a friend's combination garage/used car lot, Mike gets involved with three unsavory characters who are very interested in a Jaguar sports car that has bullet holes in the back of the driver's seat with blood stains to match.
While taking a ferry ride through the New York harbor, Mike observes a young woman sobbing. He learns that she's been victimized by a carnival con artist who charges steep fees to enter a rigged talent contest. After Mike tries to intervene, the con artist is found murdered and the District Attorney can't decide if Mike or his pretty client makes the best suspect.
Mike's quest for peace and quiet is interrupted when he intervenes in a lover's quarrel fought in a swanky restaurant. The next day, Mike is hauled into court and forced to post a peace bond guaranteeing that he'll stay away from the man he fought with the previous evening. When the man turns up dead and his fiancée accuses Mike of the crime, Hammer has to go on the lam to clear himself of the frame-up.
Mike runs into an old acquaintance, the wife of an embezzler he helped send to prison, who asks for her help. When she turns up dead in her apartment, rather than in his office as scheduled, Mike investigates. He's curious why a blue-blooded lady should arrange to bury the wife of a convict and discovers that both women are involved in a shady escort service dabbling in blackmail.
A serial killer is carving up women in the neighborhood where Sam Earl, one of Mike's part-time assistants lives. The legman wants to help Mike capture the psychopath before he strikes again, but Hammer insists it's a job for the police department, until Sam's brother's fiancée is brutally stabbed.
Mike is asked to help a popular disk jockey who's been threatened by a powerful music producer to play her performers' songs or else. While Mike is trying to persuade the producer to back off, the disk jockey is brutally beaten, and Mike is fired as a result. When the DJ collapses from the injuries he's sustained, Mike decides he's been rehired and takes after the producer and her goons with a vengeance.
Mike discovers his girlfriend's roommate has been murdered and launches his own investigation. He discovers that she was killed for a diamond she was trying to fence and tracks the murderer to the Brussels World Fair.
A convict on death row confesses to a murder that another man was executed for three years earlier and threatens to supply proof after he dies on the electric chair. Mike doesn't pay much attention to man's story until the gun used in the crime is produced. Captain Chambers is worried that an innocent man lost his life based on shoddy police work, but Hammer is convinced that the grocery store robber had a partner and sets out to prove it.
Mike is hired by a beautiful heiress to break up her father's romance with a much younger woman. Mike goes to the woman's apartment and, after interviewing her, decides it really is a May-December romance and determines to give up on the assignment. When a thug claiming to be Mike Hammer threatens the lovely lovebird with bodily harm if she doesn't give up her sugar daddy, Mike gets back on the case and investigates who would really benefit if the romance were scotched.
Working for the insurance company, Mike assists the police investigate an armed robbery of an armored car company. His investigation reveals that the sister of a crippled friend who only helped the hold-up gang because of the beatings they had given her brother. Mike can't capture the thieves without implicating the young woman.
A rich industrialist wants to hire Mike to beat up his son and heir because the young man has become engaged to woman considered beneath the family's standing. Mike turns the job down, but agrees to drive the industrialist to the family's cabin in the Catskills where the son and his fiancée are waiting out a storm. When Mike arrives, he finds the young woman dead, bloodstains on the floor and conflicting stories about how both occurred.
Mike's girlfriend, Jackie, a dancer in a chorus line, panics when a sleazy reporter points out the similarities between a pair of chorus line murders and the Jack the Ripper killings 70 years ago - both of the murdered girls were redheads and Jackie is the only redhead left in the troupe. Mike has two suspects - a doorman who hates actresses and the reporter who may be making the news instead of simply reporting it.
Mike is hired to track down a man's missing girlfriend. The client tells Hammer that he just returned from South America, but his pasty complexion belies his claim of three years in the tropics. Hammer learns that the man is really an ex-con out on parole. Mike can understand his client hiding his criminal past, but what he really can't understand is why a powerful television anchor like Butler Tilton would hire an inept private eye like Andy Anderson to tail the former bank robber.
Mike is hired by an art student's parents who are concerned when she stops writing. The artist's friends and instructor aren't concerned that she's missing, but Mike connects the girl's disappearance with the copies of the Old Masters she'd been making and a forgery that covered the theft of a masterpiece from a major art museum.
A man Mike helped get paroled from prison gets an anonymous letter accusing his wife of infidelity. Mike learns he's bought a gun illegally and tries to convince the ex-con's spouse to quit her glitzy job which requires her to be seen socially with a magazine editor known to have a roving eye before there's bloodshed. When the wife is found murdered and his friend's gone on the lam, Mike attempts to prove that the gun his friend bought was a different caliber than the one used to commit the crime.
Mike decides to investigate when the darkroom belonging to his girlfriend's brother is destroyed by someone looking for an incriminating photograph. Since the negative and all the copies of the photograph were destroyed Mike doesn't have a leads to go on, until he hires a fingerprint expert to dust the nightclub where the photographer worked. He learns that the man very interested in not having his picture taken was a racketeer who had been declared dead three years earlier when the plane he purportedly was flying in crashed and burned.
Hammer is hired by Susan Barlow but she stands him up. He investigates and runs into her husband Hank Barlow. After a fight, Hank hires Hammer to find his wife who has been missing for several days. Hammer talks to Shelton Dean of the brokerage firm Haskell & Dean, who had called Mrs. Barlow. Haskell is a recluse and Mrs. Barlow is claiming to be his daughter.
While getting a shoeshine in the Bowery, Mike spots a beautiful, well-dressed blonde who is as out of place as a field of wildflowers growing out of a crack in the concrete. When the woman is mugged in an alley, Mike comes to her aid and learns that she's searching for her husband, a blues piano player, who hasn't written to her in Wyoming in over a month. Mike traces the missing husband to a penny arcade and then follows the bodies to a fashionable apartment uptown.
Doc Fawcett hires Mike to find the person to whom he's paying protection money. He's received letters that threaten his daughter, a high-wire acrobat, will suffer a serious, if not fatal, accident unless $500 is paid every month. When the leading suspect, Lou Lindsay, is shot with Doc's gun, Mike has no shortage of suspects - Doc hated the man because he made unwanted advances towards his daughter; the girl's would-be boyfriend had been severely beaten by the roughneck, Lou had made loved than dumped have the women in Doc's carnival and the cuckolded husband of one of...
Mike is hired to locate a young woman who moved to New York, but suddenly stopped writing home to her parents. Mike quickly learns that she was killed in a hit and run automobile accident, but he can't help but wonder how the young woman went from skipping out on an apartment owing two months rent live at a fashionable flat with a wad of money in her purse at the time of her death. His investigation shows that she was making ends more than meet by working as a call girl and setting up clients for a team of muggers operating out of school for actors.
Mike is hired to find the culprit behind a series of pranks at an exclusive girls' prep school - students have been poisoned, burned with acid and now shot at. Mike has three suspects: an elderly caretaker who was forced to retire, a middle-aged administrator who was passed over when the new head mistress was hired by the school's Board of Trustees, and lovely student who craves attention.
Mike receives a desperate letter from Harris Stevens who claims that he's being held against his will in a mental hospital. The physician in charge refuses to allow Mike to see the patient, but Mike learns from a nurse that she suspects the charges are true. Mike's chances of springing his client take a turn for the worse when the nurse and the physician are found stabbed to death by a knife that Harris was making to use in a desperate escape attempt.
A friend from the old neighborhood asks Mike to travel to Chicago to claim the body of her son. While visiting the Windy City, Mike learns that the young man was murdered and the police don't appear to be interested in investigating the crime. When Mike visits the murdered man's room to pick up his personal effects, he is attacked and brutally beaten by a pair of thugs who want to know where "it" is. Mike has no idea, but he's determined to find out and make the two goons who attacked him which they'd never been born.
A pretty secretary swims across the lake to the cabin Mike has rented for vacation and invites him to a party at her uncle's estate. The party soon loses its appeal when Mike bumps into an old adversary, a lawyer who claims Mike railroaded his client by faking evidence. When the lawyer is accused of murdering his host, Mike shows the value of a fake identification to smoke out the real killer.
A young, recently married woman hires Mike to find her husband, who disappeared after visiting his estranged mother at her Berkshire Mountains mansion. Mike's efforts to visit the man's mother meet resistance from the woman's secretary and groundskeeper, but he manages to confront her in the estate's garden. He discovers that the man's mother is a former Broadway star whose face was disfigured by a knife-wielding thug ten years earlier. When he finds the body of a young man in a shallow grave on the property, he becomes more determined than ever to have the bride meet...