When the author, William Goldman, is ten years old, he comes down with pneumonia. He spends ten days in the hospital and when he comes home, Goldman's father, a Florinese immigrant, starts to read him The Princess Bride, a classic Florinese adventure tale written by S. Morgenstern. The story captures Goldman's interest and he vows to share The Princess Bride with his own son when he has one. Unfortunately for Goldman, his son, Jason, is overweight and has no sense of humor. Regardless, while Goldman is in California working on the screenplay for The Stepford Wives, his wife Helen reminds him that it's Jason's birthday and Goldman spends two hours and several hundred dollars on the phone to get a copy of The Princess Bride delivered to Jason.
When Goldman gets home, he discovers that Jason was only able to get through the first chapter. This disturbs Goldman, as he remembers the book as being extremely exciting. When he opens Jason's copy to revisit the story, he realizes the problem: S. Morgenstern's novel is 1000 pages of satirical Florinese court history, and Goldman's father only read him the exciting passages. Goldman starts calling around to get permission to abridge Morgenstern's classic, which is the tale to follow. He's cut out almost 700 pages of court history and distilled it down to the "good parts."
The story follows the very beautiful Florinese milkmaid Buttercup. Buttercup doesn't care about being beautiful and instead spends her time riding her horse and bossing around the farm boy, Westley. As she gets older men begin to take notice of her beauty and one day, Count Rugen and his wife, the Countess, stop in at the farm. While they're there, Buttercup notices that the Countess seems interested in Westley. By evening, Buttercup is extremely jealous and the next morning, she confesses her love to Westley. He slams his door in her face but that evening, he comes to say goodbye: he's going to America to seek his fortune so that they can live happily together. According to Morgenstern, their parting kiss is the most perfect kiss ever. Buttercup starts to attend to her appearance in Westley's absence, but not long after he leaves, she receives news that the Dread Pirate Roberts attacked Westley's ship and that he's certainly dead. The pain of losing Westley makes Buttercup the most beautiful woman in the world.
Meanwhile, at the Florinese court, Prince Humperdinck is annoyed to learn that since his father, King Lotharon, isn't well, he needs to marry so that he can become the king. Humperdinck loves hunting more than anything, so taking time away from his beloved Zoo of Death, which houses animals for him to hunt, is obnoxious. After a failed attempt to court Princess Noreena from the neighboring country of Guilder, Count Rugen introduces Humperdinck to Buttercup. Buttercup agrees to marry Humperdinck, but says that she can never love him. She spends the next three years going to princess school. About 100 days before her wedding, however, while on her daily ride, three criminals—Vizzini, a Sicilian hunchback; Fezzik, a Turkish giant; and Inigo, a Spanish swordsman—capture her.
As Buttercup's captors reach the Cliffs of Insanity, they notice that a man in black is following them. Vizzini deems this "inconceivable." Vizzini, Inigo, and Buttercup tie themselves to Fezzik as he starts to climb the Cliffs using a rope. Vizzini bullies Fezzik into going faster, especially as the man in black starts to climb up after them. Vizzini finally decides to leave Inigo to deal with the man in black and continue on with just Fezzik and Buttercup. Morgenstern offers a bit of Inigo's backstory: as a child, a six-fingered nobleman killed his father, Domingo Montoya, and he's dedicated his life to finding and killing this man. He's a "fencing wizard," and Vizzini saved him from life as a drunk. When the man in black reaches the top of the cliffs, the men duel. The man in black wins, knocks out Inigo, and runs after Vizzini.
Vizzini then leaves Fezzik behind to throw a rock at the man in black. Fezzik's parents taught him to fight as a child, even though he hates fighting and really loves rhymes. More than anything, he hates being alone, so he'll do whatever Vizzini asks of him. Fezzik challenges the man in black to a fair fistfight, which the man in black wins. The man in black then comes upon Vizzini and challenges him to a duel of wits. He puts lethal iocane powder in a wine goblet and asks Vizzini to choose the goblet that's not poisoned. Vizzini cheats but dies anyway; the man in black reveals that he's immune to the poison and both goblets were poisoned.
The man roughly drags Buttercup through the woods for hours until they get to the top of a ravine. There they fight and notice that Humperdinck is after them. Buttercup takes her chance and shoves the man down into the ravine, at which point she realizes he's Westley. She throws herself down after him. At this point, Goldman cuts in and says that Morgenstern didn't write a "reunion scene" between the lovers at the bottom of the ravine. Because this is an abridgement, Goldman's editors wouldn't let him write his own and include it—but if the reader writes to the publishing house, the publisher will mail them Goldman's reunion scene.
Using his skill at hunting, Humperdinck tracks Westley, Buttercup, and Vizzini's crew all the way to the top of the ravine, at which point he sees that Westley and Buttercup are headed straight for the dangerous Fire Swamp. The Fire Swamp is a terrifying place, so in order to distract Buttercup, Westley tells her how he's still alive. He is now the Dread Pirate Roberts; many men have played Roberts over the years to take advantage of the name recognition. Buttercup falls into Snow Sand and R.O.U.S.s (rodents of unusual size) attack Westley, but they make it out of the Fire Swamp. On the other side, Humperdinck and Rugen are already waiting for them. Buttercup asks Humperdinck to release Westley in exchange for her hand in marriage. He agrees, but secretly asks Rugen to put Westley in the fifth level of his Zoo of Death. Meanwhile, Inigo goes back to the Thieves Quarter in Florin City, and Fezzik panics and forgets what he's supposed to do.
As the 90 days of wedding festivities begin, Buttercup begins having nightmares and decides she can't marry Humperdinck. He promises to send four ships to find the Dread Pirate Roberts’s ship and see if Westley will come back for her. He reveals to the reader that he's not going to follow through, as he wants to murder Buttercup and blame it on Guilder so that he will have a reason to start a war. Humperdinck and Count Rugen use what Buttercup shares about Westley to torture him down in the Zoo of Death once the zookeeper, the albino, returns Westley to health. Westley knows how to "take his mind away," so the torturing does nothing to him. This changes when Rugen begins using his Machine on Westley. Though the Machine looks ridiculous, it painfully sucks away years of Westley's life.
A few days before the wedding, Humperdinck tells Yellin, the Chief of All Enforcement, to clear out the Thieves Quarter so that no Guilderian spies can attack Buttercup. Yellin forms a brute squad that includes Fezzik. As the brutes capture the last of the criminals, Fezzik finds Inigo drunkenly refusing arrest. Fezzik returns Inigo to a sober state and shares that Vizzini is dead and he found the six-fingered man: it's Count Rugen. Inigo reasons that Westley is smart enough to plan the attack on Count Rugen and, knowing Rugen and Humperdinck, he thinks they're likely torturing Westley.
At this moment, they hear a scream of "ultimate suffering": it's Westley. Humperdinck, in his anger with Buttercup, used Rugen's Machine to suck all the life out of Westley. When the albino emerges from the Zoo of Death to look for a wheelbarrow to deal with Westley's body, Inigo and Fezzik knock him out and begin descending through the levels of the Zoo. It's a terrifying experience. Inigo is distraught when he finds Westley dead, but he and Fezzik take Westley to Miracle Max, King Lotharon's old miracle man, to purchase a miracle. Though Max tries to refuse the job, he agrees to do it when he learns that bringing Westley back to life will humiliate Humperdinck.
After giving Westley the resurrection pill, he begins to come to life slowly. At the same time as he, Inigo, and Fezzik storm the castle, Humperdinck and Buttercup get married in a private ceremony. Inigo pursues Count Rugen and successfully cuts out his heart, while Westley stops Buttercup from committing suicide and tricks Humperdinck into putting down his sword. Fezzik appears outside with four white horses and the heroes run away on them. Goldman explains that his father left the story there, but Morgenstern writes that as the heroes escape, things start to go wrong immediately and Humperdinck starts to chase them. Goldman’s actual novel The Princess Bride itself ends there.
In both the introduction to Buttercup's Baby, Morgenstern's sequel to The Princess Bride, and in the introduction to the 30th anniversary edition, Goldman discusses the process of turning The Princess Bride into a movie and how successful he was in that endeavor. However, when his grandson Willy is about seven and asks Goldman to read him Buttercup's Baby, Goldman learns that the movie was almost too successful: in part because of all the lawsuits brought by the Morgenstern estate and in part because the movie was more successful than Goldman's abridgement, the Morgenstern estate wants Stephen King to abridge Buttercup's Baby. King eventually agrees to let Goldman abridge the first chapter of Buttercup's Baby if he promises to go to the Morgenstern Museum in Florin and research it properly.
Buttercup's Baby begins with a scene of Fezzik chasing a skinless madman, who has Buttercup's baby daughter, Waverly. He throws himself off a cliff after the madman tosses the baby. Goldman thinks this beginning is awful, but can't fix it since it's an abridgement. The second part of the first chapter is what Goldman calls the Unexplained Inigo Fragment. It does nothing for the plot, but it flashes back ten years to when Inigo was studying in Italy and fell in love with a young Countess. Then, the story skips ahead in time to the end of The Princess Bride. It details all the things that go wrong with the heroes' escape and explains how Westley manages to get his friends through a dangerous whirlpool to the secluded One Tree Island. There, they return to health and Buttercup gets pregnant. After 50 hours of labor, something "invades" Fezzik's mind and he performs a C-section. He and Waverly become best friends, and this is why he leaps off the cliff after her. Goldman then thanks the reader for joining him on this journey and getting so involved in the story. He hopes that Morgenstern gives the heroes a happy ending.