The last chapter of the novel begins with Oedipa's return to Echo Courts, the hotel in San Narciso, where she once again finds The Paranoids hanging around by the pool. Serge sings a song about Humbert Humbert, the main character in Vladimir Nabokov's Lolita. Serge then tells Oedipa how Metzger and Serge's own ex-girlfriend eloped to Nevada after Metzger turned over all his legal responsibilities related to the Inverarity Estate to a different lawyer at the legal firm.
Oedipa calls Professor Emory Bortz at San Narciso College and gets his wife on the phone. Mrs. Bortz invites Oedipa to stop by sometime to speak further with her husband about Wharfinger and Jacobean drama. On her way to his house, she drives by Zapf's Used Bookstore, where she first bought the paperback edition of The Courier's Tragedy, and she sees that it has completely burned to the ground. The man in the government surplus store next door tells her that the store was burned down by an arsonist anxious to scam money from an insurance company. Oedipa eventually arrives at Bortz's house, where she finds him in the backyard with a group of his students. He knows a good deal about the Tristero version of The Courier's Tragedy, a version he describes as a "pornographic" rendition found only in the Vatican. He claims the version was a propaganda piece used by a group of ultra-devout Puritans. Bortz ultimately states, however, that the person who understood Wharfinger best was Driblette, who, tragically, committed suicide only a couple of days earlier. Oedipa will never know why Driblette included the lines about the Tristero in his own version of the play.
Bortz shows Oedipa slides of the Vatican Copy of The Courier's Tragedy and explains that the mention of Tristero may have been added in by a Puritan group (the Scurvhanites) who wanted to deliver a particular moral message related to the "Other" that transcends even God. Bortz also gives Oedipa a copy of An Account of the Singular Peregrinations of Dr. Diocletian Blobb, the eighth chapter of which tells the story of one of Blobb's trips in a Torre and Tassis coach that was ambushed by Tristero riders in black capes. Only Blobb and his servant survived, and the cloaked men sent him back to England to warn the English of the might of the Tristero.
Piece by piece, Oedipa learns how the Tristero was created around the year 1577 in the Netherlands. After William of Orange achieved independence from Spain and the Holy Roman Empire, he replaced Leopold I, who owned the Taxis postal monopoly, with a man named Jan Hinckard. However, Hinckard was challenged by his cousin Hernando Joaquin de Tristero y Calavera who claimed to be the rightful heir of the family fortune. Tristero believed himself to own everything Hinckard owned, including his job. Tristero fought a guerilla war against his cousin from 1578 until 1583, a time during which the postal operation fell into a state of instability. Eventually, Tristero gave up fighting and set up his own covert system. Oedipa finds out later on in the chapter that the Thurn and Taxis system struggled throughout the 17th century, and Oedipa believes that that must have been a period of strength for the Tristero. Bortz forwards the theory that toward the end of the 30 Years' War (which ended in 1648), somebody may have tried to merge the two systems but failed. Oedipa finally finds out that the French Revolution saw the end of the Thurn and Taxis monopoly. She wonders if perhaps Tristero won out in the end.
Oedipa spends the next couple of days going to libraries in between meetings with Bortz and Genghis Cohen. She also goes to Driblette's funeral, where she realizes that she is completely frustrated by her inability to find out why he said those two lines about the Tristero in his performance. Slowly, Oedipa begins to give up.
Much of this final chapter is a long account of the history of the whole Tristero system. Indeed, Pynchon has gone to great lengths to recreate fully the story of how this Tristero system came to be. Of course, most of this history is entirely fictional. Nevertheless, the question of who or what Tristero is has more or less been answered. The problem, however, is that even though this big chunk of the mystery has been solved, it does Oedipa virtually no good in finding out what she wants to know. The real problem to Oedipa is why Driblette referred to the Tristero in his production of The Courier's Tragedy, but once again, a communication breakdown--Driblette's death-- prevents her from ever finding out. She knows that part of her own mystery can never be solved now. Once again, this ties in to the book's theme of the problems of communication.