Six months after the disappearance of family man Tom Gruneman in New York City, the police have been unable to locate the missing man. Their only lead is an obscene letter written by Gruneman to a New York call girl named Bree Daniel, leading Lt. Trask, the officer on the case, to posit that Gruneman was living a double life. Gruneman's wife and Peter Cable, an executive at the Pennsylvania research company at which Gruneman was employed, hire Pennsylvania police officer and Gruneman family friend John Klute to investigate the missing man’s disappearance. Trask gives Klute Bree’s address, along with the information that they arrested her for prostitution two years earlier. In exchange for a reduced sentence, Bree had answered their questions about Gruneman, but was unable to identity his photo or remember anything about her liaison with him, and claims she had been receiving “breather” phone calls ever since. Klute rents a room in the basement of the run-down brownstone in which Bree lives and proceeds to tape record her phone conversations. One morning, after Bree was awakened by a breather call the previous night, Klute rings her doorbell and identifies himself as a private investigator who would like to talk to her about Gruneman. After closing the door in his face, Bree goes to visit her psychiatrist, whom she has been seeing in attempt to relinquish her life as a call girl. Bree, who is trying to pursue a career in modeling or acting, has just come from a humiliating interview with a casting agent, while on the previous day, she had been overlooked and dismissed at a modeling agency. Bree confides to the psychiatrist that she enjoys being a call girl because she is always in control of the situation, whereas in modeling or acting, someone else is calling the shots. Klute begins to tail Bree, following her to a warehouse in the garment district, where she has an appointment with an elderly client, Mr. Goldfarb, who owns the business. Bree arrives wearing an alluring, slinky gown, and as Goldfarb sits at his desk and watches, she sensuously removes her garments while weaving a story about a romantic interlude with an elderly, worldly man. Upon returning home, Bree is greeted by Klute standing on the stairs of her building. When he shows her his room, decorated only by a single trundle bed, his tape recorder and the photographs of Gruneman and Bree that Klute has pinned to the wall, Bree thinks he is trying to blackmail her and becomes defensive. After he assures her that his only motive is to find Gruneman, Bree invites him upstairs to her apartment where she reiterates that she does not remember her meeting with Gruneman but recalls being hired by a sadist who beat her up and tried to kill her. She then tries to seduce Klute into giving her the tapes, but he resists her. Their conversation is interrupted by the phone ringing, but when she answers, no one is on the line. Hearing noises on the roof, Klute goes to investigate, and although he pursues the intruder into the cellar, the man escapes. When Klute returns to her apartment, the shaken Bree tells him that Frank Ligourin, her former pimp, arranged the date with the sadist. As Klute sits in a chair guarding Bree that night, Cable sits in his posh offices listening to a recording he taped of Bree’s seductive talk on the night of their date. The next morning, Bree takes Klute to meet Frank, who informs him that Jane McKenna, one of his prostitutes who was jealous of Bree, arranged Bree’s date with the sadist. Jane has since committed suicide, and now Arlyn Page, who met the man and worked for Frank until she became a junkie, is the sole link to the sadist. After leaving Frank’s, Klute gives Bree the tapes he has made of her, which she bitterly tosses into a trash can then stalks off. Later, Klute asks Bree to help him find Arlyn, and they pore through grisly police photos of murdered women. That night, Bree hears creaking coming from the roof and terrified, knocks on Klute’s door. He offers her his bed, pulling out a second mattress for himself. As Klute sleeps, Bree climbs into his bed and seduces him. The next morning, Klute is baffled by the development in their relationship until Bree announces that she did not have an orgasm because she regards him only as a “john.” They continue their search for Arlyn, descending a hierarchy of prostitution, from an exclusive brothel to cheap street walkers, finally locating her in a tenement where she and her boyfriend are anxiously awaiting their drug dealer. When Klute shows Gruneman’s photograph to Arlyn, she says the man she met was much older. Shaken by the degradation to which Arlyn has sunk, Bree runs away from Klute and goes to a dance club where, after flirting with one of the customers, she settles in with Frank. Klute follows her there, and when she sees him, she nuzzles Frank, after which Klute leaves in disgust. Klute then reports his findings to Cable, stating that Arlyn has verified that Gruneman was not the man who beat Bree and that he intends to pursue Arlyn until she identifies the sadist. Klute then goes to Bree’s apartment, where he finds her disheveled and unstrung. He gently puts her to bed and when she has a nightmare, comforts her. Rankled by Klute’s solicitude, Bree confides to her psychiatrist that although her relationship with Klute engenders sexual feelings she has never felt before, she is hoping it will end so that she will once again be in total control. When Arlyn is murdered, Klute, surmising that her murderer knew Gruneman, asks Trask to investigate everyone connected to Gruneman. The police then compare the type style of the obscene letter with the type styles used by Gruneman and his friends and reach the conclusion that Cable wrote the letter. To set a trap for Cable, Klute meets with him and asks for $500 to purchase Jane’s address book. Promising Klute the money, Cable takes off in a private helicopter for a business meeting. Upon returning to Bree’s apartment, Klute is shocked to find Frank there, waiting for Bree to finish packing her suitcases and leave with him. Klute jealousy lunges at Frank and beats him up, after which Bree lashes out at Klute with a pair of scissors, slashing his sleeve. After Klute wordlessly turns and leaves, Bree, unable to deal with the extremity of her feelings for him, goes to see her psychiatrist but finds the doctor is not in. Needing to talk to someone, Bree phones Goldfarb and arranges to come to his office. When she arrives, however, she finds that Goldfarb has gone home because he was not feeling well and that the office is about to close. Bree, upset because Goldfarb assumed that she came to see him on business when she really wanted to talk to him as a friend, asks to stay behind to leave a message for him, unaware that Cable followed her there. Meanwhile, Klute has learned that Cable missed the plane to his business meeting, and learning from the psychiatrist’s secretary that Bree has left a message for him to call her at Goldfarb’s, he hurries there. As Bree paces around the racks of clothes, she is confronted by Cable, who admits that he murdered Gruneman after Gruneman discovered that Cable accidentally killed Jane while administering a beating to her. Expressing contempt for prostitutes because they prey on the sexual fantasies of others, Cable plays the tape he recorded as he bludgeoned Jane to death. Titillated by Bree’s sheer horror, Cable switches off the tape recorder and lunges at her just as Klute arrives and charges at Cable. While stepping backward to avoid Klute, Cable plunges out a window to his death. Some time later, a still ambivalent Bree packs her suitcases to leave New York with Klute. As she is about to walk out the door, the phone rings and after informing her caller that she is leaving town and not coming back, she hangs up the receiver.