When Jack Williams, a former New York cop who has lost an arm in World War II saving his friend Mike Hammer's life, falls in love with Myrna Devlin, a young heroin addict whom he stops from jumping off a bridge to commit suicide, he asks Manning to admit her to her clinic for psychotherapy. After Myrna has become clean, she and Williams become engaged, and the couple keep up a casual friendship with Charlotte Manning. This is how Williams's growing suspicions about Manning's business lead him to privately and secretly investigate even further into the matter. When he realizes that Hal Kines, one of Manning's college students who has spent some time at her clinic and who has become one of her casual acquaintances, is in fact a criminal, he wants to talk to her about it and tells her so. When, at a party given by Williams in his apartment, Charlotte Manning sees some old college yearbooks whose contents (and photos), if made public, would expose Kines's double life, she has to act fast. After the party, she goes home but on the same night, undetected by Kathy, her black maid, goes back to Williams's apartment (Myrna, his fiancée, does not live there) and shoots him in the stomach using a silencer. She does so in a particularly sadistic way, watching him die slowly. Then she takes the college yearbooks and leaves.
None of the guests at Williams's party has a Watertight alibi, but to both Pat Chambers, the cop investigating the murder, and Mike Hammer, a friend of his and private investigator, none of them has a motive either. Throughout the book, as more and more immediate suspects are eliminated (shot) ("If this kept up there wouldn't be anyone left at all."), Hammer briefly ponders the question if the killer could be an "outsider" — someone wholly unrelated to the group of people who have been at Williams's party, for example someone Williams was after in his capacity as investigator for an insurance company. Chambers also thinks along similar lines: Williams's (secret) connection with Myrna's former drug dealers might have cost him his life. But they soon abandon that theory.
When Mike Hammer sees Williams's body ("For the first time in my life I felt like crying"), he makes a solemn vow: He promises that he will find the murderer and execute him himself, avoiding the U.S. judicial system altogether. He says that if he left it to the courts to punish the perpetrator, some clever lawyer would surely achieve an acquittal and the murderer would get away with his crime. This is why he himself will be the jury – and the judge, for that matter. Throughout their basically separate investigations, Hammer and Chambers work closely together, exchanging information and evidence. But each of them hopes he will be the one to find the killer in the end.
The immediate suspects Hammer finds himself confronted with are:
It takes Hammer and Chambers a relatively long time to figure out what is really going on. In the meantime, Charlotte Manning, unsuspected by everybody, continues bumping off those who have become dangerous for her. At the same time, her relationship to Hammer deepens. What really goes on inside her head is difficult for the reader to fathom, as we see everything through Hammer's eyes, and for a very long time he is completely blind to the facts ("I hope you get him,' she said sincerely."). During a walk through Central Park, while Charlotte Manning is baby-sitting for one of her female friends, she and Hammer are shot at — and in broad daylight, too. The driver of the car and sniper is George Kalecki, but it does not become clear until much later that he was after Manning rather than Hammer. He misses, though.
On a Saturday morning, Hammer picks up Myrna Devlin and gives her a lift. They drive to the Bellemy twins' estate in the country for a gigantic all-day party there. Charlotte Manning says she has some business to attend to and will be there in time for a tennis game due to take place that evening. After an unsuccessful attempt at playing tennis himself, Hammer gets rid of his sleep deficit by spending all day in his room, fast asleep, with "old junior" — his gun — close to him. He is woken up just in time for dinner, during which Harmon Wilder, the Bellemys' lawyer, and Charles Sherman, Wilder's assistant, are pointed out to him. This is a fine — and the final — distractor in the novel: Wilder and Sherman are suddenly missing from the party after Myrna Devlin has been found shot. In fact they had illicit drugs on them and did not want to be found out. During the tennis game, Mary Bellemy asks Charlotte if she can "borrow" Hammer. Then she leads him into the woods where, in complete darkness, she strips in front of him. When Hammer realizes what she has done it is too late: After all, he is just a man. They have sex right then and there. They return to the party just as a maid discovers Myrna's body in an upstairs room, in front of a large mirror. Both Pat Chambers and the local police are called in, and the alibis of each of the guests — more than 250, including gatecrashers — are checked. Again Charlotte can convince everyone that she could not have done anything.
Back home, Hammer retreats into his apartment to think. He does so all Sunday, unkempt and unshaven. ("Every ashtray was filled to overflowing.") Finally, he knows the identity of the killer. This is when he goes to Charlotte's place, recapitulates the whole crime and finally shoots her dead, despite her efforts to distract him by stripping from her own efforts to grab her gun and pull the trigger on him.
Viewed from an early 21st century point of view, next to nothing about Mike Hammer's character is politically correct: Time and again the reader is confronted with his an eye for an eye mentality. Just like Philip Marlowe (cf. the opening pages of Raymond Chandler´s The Big Sleep), he fights on the right side but "tests very high on insubordination". He has not forgiven the Japanese for attacking Pearl Harbor and maiming his friend. In his favourite haunt, hard-drinking Hammer asks the waiter to bring him "a rye and soda every fifteen minutes". And finally, his relationship with women is highly ambiguous. He feels that "there's lots of dames I could park with if I felt like it". The games Hammer plays with Velda, his secretary, a licensed private investigator in her own right, are bound to be criticised: He never makes a pass at her, but he knows that she is in love with him and would marry him at once ("How I hated to tell Velda about Charlotte!"). How immoral (amoral?) is Hammer? On the one hand, he refuses to sleep with Charlotte. On the other, he willingly yields to a promiscuous woman like Mary Bellemy — twice — even if he tells her that the first time was "a mistake". All in all, it probably is exactly this macho image, more than anything else, that readers today find appealing.