Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Initial Situation
At the start of the book, Oedipa Maas learns that she has been named executrix of the estate of her ex-boyfriend Pierce Inverarity. Oedipa wonders why she has been named, but is not overly bothered by it or interested in the business of carrying out Inverarity's final wishes. She seems basically content with her life in Kinneret, living with her husband Wendell "Mucho" Maas, a local DJ.
Conflict
Oedipa decides to drive down to San Narciso to meet with Metzger, the lawyer who has been assigned as co-executor of Inverarity's estate. The two end up sleeping together at Oedipa's hotel, and she officially becomes tangled up in the business of Inverarity's estate. A few days later, they go to Fangoso Lagoons and she hears a bizarre story about Inverarity buying the bones of dead American GI's without paying for them.
Complication
Before going to see Wharfinger's play The Courier's Tragedy, Oedipa has only odd coincidences to go on. She knows of the American GI's; she has seen the muted post-horn and the WASTE symbol; and Mr. Thoth told her the story of his grandfather (a Pony Express rider attacked by men in black pretending to be Indians in the late nineteenth century).
Oedipa sees that the play is full of odd resemblances to the events she has been hearing about in real life, and, at the play's end, the main character puts a name to what (might) link it all together: Tristero. Oedipa's hooked.
Climax
Things escalate super-quickly. Oedipa goes to San Francisco looking to escape the signs of Tristero that she suddenly is seeing everywhere around her. Instead of escaping, she becomes totally immersed in it. It's not clear if Oedipa is hallucinating, but literally everywhere she looks she sees Tristero. At the end of the night, she follows a Tristero postman and is eventually led back to the house of the mad scientist John Nefastis.
Suspense
At this point, Oedipa is scared that she is losing her mind. She goes to her psychotherapist, Dr. Hilarius, only to find out that he has gone insane and that he was a Nazi all along. She then meets up with her husband, "Mucho," only to find that Hilarius has been feeding him hallucinogenic drugs and that he's pretty much lost it.
She returns to the motel Echo Courts, where she learns that her lawyer boyfriend Metzger has run off with a younger girl, and then to top it all off she learns that the director of The Courier's Tragedy killed himself by walking into the Pacific Ocean. Oedipa is completely isolated and alone.
Denouement
Oedipa begins to lose interest in Tristero, even though more clues begin to appear and the puzzle starts to fit together. Working with the Wharfinger scholar Emory Bortz, she pieces together that the Tristero arose in the late sixteenth century and took up a guerrilla campaign against the dominant postal system of the Holy Roman Empire, Thurn and Taxis.
But Oedipa becomes more and more reluctant to ask about Tristero. In a fit of desperation, she gets drunk and goes driving on the highway with her headlights off.
Conclusion
Genghis Cohen, the stamp expert, reveals to Oedipa that there is a mysterious bidder trying to buy up all the stamp forgeries from Inverarity's estate (which will be auctioned off as "Lot 49"). He believes the man comes from Tristero.
Oedipa attends the auction with him, only half-interested, but wondering if the "crying of lot 49" will be the moment of revelation that she has been looking for, or just another clue in an endless mystery. We never find out which it is, as the book ends just before the stamps are auctioned off.