Cleland's book consists of a series of letters from Frances "Fanny" Hill to an unknown woman, in which the protagonist explains and justifies her life-choices. Fanny Hill is a young, poorly-educated girl who lives near Liverpool. After her parent's death, a friend convinces the girl to move to London but soon disappears. Fanny is hired as a maid by Mrs. Brown, a woman she believes to be a wealthy lady but turns out to be a brothel madam.
Mrs. Brown plans to use the girl as a prostitute, but Fanny finds the man she is expected to give her virginity to repulsive and fights him off. Fanny falls ill for several days, and Mrs. Brown decides to sell Fanny's sexual favors to another wealthy gentleman. Spying other prostitutes having sex with clients and speaking with her colleague Phoebe, with whom she engages in mutual masturbation, Fanny grows impatient to have sex with men.
Soon afterward, the girl meets a young nobleman, Charles, who helps Fanny escape the brothel and takes her to his flat. They became lovers, and for many months they meet almost daily to have sex, while Fanny tries to improve her manners, After eight months, Fanny becomes pregnant, and Charles mysteriously disappears: he has been sent by his father to the South Seas to win a fortune. Fanny miscarries and falls ill; she is nursed back to health by the landlady Mrs. Jones.
Mrs. Jones eventually employs Fanny as a prostitute, and introduces her to the wealthy Mr. H. Fanny becomes his mistress, and realizes sex could be done just for pleasure. Out of jealousy, she seduces Mr. H's servant, gets caught and loses the man's support.
Fanny is taken in by Mrs. Cole, who runs a brothel in Covent Garden, and meets three other prostitutes (Emily, Harriet, Louisa), who briefly outline their backgrounds. The girls engage in many sexual relationships with men of all types- those who are impotent, masochists, perverts and so on. Fanny also meets a bisexual young man and is startled by her first encounter with male homosexuality. In a tavern, she surprises two young men engaging in anal sex in the next room and faints. Later, she rouses the villagers to try to hunt the two men down and punish them.
Fanny's colleagues follow different paths: Louisa seduces a brutal teenager and leaves Mrs. Cole's brothel because of his affair with another young man. Emily, invited with Fanny to a country estate, is rescued by her parents who, unaware of her career as a prostitute, want her to come home again. She accepts.
Mrs. Cole retires, and Fanny starts living off her savings. She meets a man who falls in love with her but treats her like his daughter. He dies and leaves his small fortune to her. Therefore, Fanny is now able to try to locate Charles, who disappeared in the South Seas. Several months later, she runs into Charles in a tavern: he had been shipwrecked on the coast while coming back to England. The couple make love several times and, despite Fanny's confessions about her life of vices, Charles forgives her and asks the girl to marry him. She complies heartily.