For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I:
Oedipa only becomes interested in Inverarity's estate gradually. But when she goes to Richard Wharfinger's play The Courier's Tragedy, she hears the name of a group that might explain all of the bizarre things she's been hearing. The name is "Tristero." This is Oedipa's point of no return. From here on out, she becomes totally obsessed with solving the mystery of the Tristero.
Act II:
At no point is Oedipa farther from solving the Tristero mystery than when she gets drunk and drives her car on the highway with the lights off. By this point, her psychotherapist has gone insane, her husband has lost his mind to hallucinogenic drugs, her new lover has run off with a younger girl, and the director of The Courier's Tragedy has killed himself.
Oedipa can't tell if Tristero is real, or if she is insane and has imagined it, or if somehow her ex-boyfriend Pierce Inverarity built an enormous practical joke into his estate and it's all a hoax. Everything is uncertain.
Act III:
As the book approaches its conclusion, more and more details about Tristero emerge, but the mystery seems no closer to being solved. Oedipa herself begins to lose interest. The book ends one moment before "the crying of lot 49." Lot 49 is a collection of stamp forgeries that belonged to Inverarity and are being auctioned off as part of his estate.
They appear to have been created by Tristero, and when a mysterious bidder emerges in the days before the auction, Oedipa's friend Genghis Cohen becomes convinced that he is from Tristero. Yet we never find out if this is Oedipa's moment of revelation or just another turn in an endless maze.