Dr. Charlotte Manning is young, beautiful, blonde, and well-to-do psychiatrist motivated by greed. To increase her profit, she becomes involved with a crime syndicate with ties to both prostitution and drug-trafficking. The leader of the organization is Hal Kines, who has had plastic surgery to make him look younger, and who recruits young women for the sex trade. Manning herself has upscale clientele, though her trade is not prostitution but coercing her patients into becoming dependent upon medications, primarily heroin, in order to extort money from them. On the surface, Charlotte Manning maintains her façade as a renowned psychiatrist.
As the sinister plots of Manning and the crime syndicate evolve, Jack Williams falls in love with Myrna Devlin when he stops her from committing suicide by jumping from a bridge. Williams himself is a former New York police officer who has lost his arm in World War II saving the life of his friend Mike Hammer. Williams asks Dr. Manning to admit Devlin to her clinic for psychotherapy. After Myrna has become clean, she and Williams become engaged, though the couple maintains a casual friendship with Manning. Over time, Williams becomes suspicious of Manning's business, and secretly investigates further. 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. When, at a party given by Williams in his apartment, Charlotte Manning finds old college yearbooks whose contents would expose Kines' criminal actions, she has to act fast. After the party, she goes home but on the same night, undetected, returns to Williams' apartment and shoots him in the stomach with a silencer as she watches him die slowly. Then she takes the college yearbooks and leaves.
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 they have sex. They return to the party just as a maid discoversMyrna's body in an upstairs room, in front of a large mirror. Both Pat Chambers and the police are called in, and the alibis of each guest is checked. Again Charlotte can convince everyone that she could not have done anything.
Back home, Hammer retreats into his apartment to think. 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 pull the trigger on him.