Oedipa Maas, the young wife of a man named Mucho, lives in Kinneret, California. One day, she receives a letter from a law firm telling her that her ex-boyfriend, Pierce Inverarity, has died and named her the executor of his estate. Oedipa resolves to faithfully execute her duty, and she travels to San Narciso (Pierce's hometown) where she meets the lawyer, Metzger, assigned to help her, with whom she spontaneously begins an affair.
As they go about sorting through Pierce's tangled financial affairs, Oedipa takes note of the fact that Pierce owned an extensive stamp collection. One night, Oedipa and Metzger go to a bar called The Scope, where they meet Mike Fallopian, a member of a right-wing fanatical organization called the Peter Pinguid Society. In the bathroom of the bar, Oedipa sees a symbol that she later learns is supposed to represent a muted post horn. Written below the symbol are the acronym W.A.S.T.E. and the name "Kirby." Oedipa makes a note of all this info before returning to chat with Mike at the bar.
Oedipa and Metzger take a trip one day to Fangoso Lagoons, an area in which Pierce owned a substantial amount of land. There, they meet a man named Manny di Presso, a lawyer who is suing the Inverarity estate on behalf of his client, who recovered and sold human bones to Inverarity but did not receive proper payment. Pierce wanted the bones to make charcoal for cigarette filters. A member of The Paranoids, a hippie band that follows Oedipa around, points out that Manny's story is similar to that of the 17th-century play The Courier's Tragedy. Oedipa and Metzger decide to see a production of the play nearby. The play mentions the word "Tristero," a word that fascinates Oedipa because of its placement within the play. She goes backstage to speak with the director, Randolph Driblette, who tells her to stop overanalyzing the play. She resolves to call him back later.
After rereading Pierce's will later on, Oedipa goes to a stockholders' meeting for the Yoyodyne company, a firm owned in part by Inverarity. After taking a brief tour, she stumbles into the office of Stanley Koteks, who is drawing the muted post horn symbol on his pad of paper. He tells her about a scientist named John Nefastis who has built a type of Mexwell's Demon, or a physically impossible machine that allows for perpetual motion by violating the Second Law of Thermodynamics. Koteks encourages Oedipa to meet with Nefastis.
Wanting to learn more about The Courier's Tragedy, Oedipa gets an anthology of Jacobean revenge plays. She notices that the paperback copy has no mention of the Tristero, however, which puzzles her. She decides to go to Berkeley to meet with the publisher. In the meantime, she stops by an elderly care home that Pierce had owned, where she meets an old man with a ring depicting the muted post horn. She also hires a philatelist (stamp expert) named Genghis Cohen to go through Pierce's stamp collection. After doing so, Genghis tells her that some of Pierce's stamps have a muted post horn in their watermark. Oedipa begins to realize that she is uncovering a large mystery.
Oedipa goes to Berkeley to meet with John Nefastis, who shows her his perpetual motion machine. It can only be operated by people with special mental capabilities allowing them to communicate with the machine, and he tells Oedipa that she has no such mental skills. He then propositions her, causing her to run out screaming. Oedipa then begins a very, very long night of wandering around aimlessly all over the Bay area. She encounters the muted post horn symbol almost everywhere, leading her to believe that she may be hallucinating. Just before dawn, however, she encounters an old man who hands her a letter and asks her to deliver it via W.A.S.T.E. under the freeway. After helping the man to his room, Oedipa finds a W.A.S.T.E. facility under the freeway, drops in the letter and waits for the delivery man, whom she follows to Oakland and back to Berkeley after he picks up the letters and delivers them. Oedipa returns to her home in Kinneret to see her doctor, who begins shooting at her as she pulls up. He has gone crazy, obsessed with the idea that Israelis are coming to kill him because he assisted the Nazis in World War II. After he is arrested, Oedipa sees her husband, Mucho, and spends some time with him, although she quickly sees that he has become addicted to LSD, making it difficult to communicate effectively.
Increasingly alone, Oedipa seeks out Emory Bortz, an English professor at San Narciso College who has extensive knowledge of Jacobean revenge plays. With his help, she pieces together the history of the Tristero, which dates back to mid-16th-century Europe. She learns that Driblette has died, which means she will never know why he included the lines about the Tristero in his production of The Courier's Tragedy (these lines are not ordinarily included in the play). Oedipa begins to give up as she realizes that she is very lonely and has no real friends. She visits Mike Fallopian again, who suggests that the whole Tristero mystery may be nothing more than a huge, complex joke played on her by Pierce. Oedipa will not accept this possibility but realizes that every route leading to the Tristero also leads to the Inverarity Estate. Meanwhile, Genghis Cohen helps her piece together some mysteries about Pierce's stamp collection, which is to be auctioned off by a local dealer as Lot 49. Genghis has heard that a secretive bidder will attend the auction to bid on Lot 49, but he will not reveal himself beforehand. Oedipa goes to the auction, excited to find out who the bidder is, thinking that he may know the key to the Tristero. The novel ends as Oedipa sits in the room waiting for the crying of Lot 49, when she will discover the identity of the mystery bidder.