The Princess Bride is the story of the beautiful maiden Buttercup and of her poor peasant lover, Westley. The two have a youthful chaste affair, then are separated by poverty. Buttercup is uncomfortably betrothed to a local prince, Humperdink, while Westley vanishes. What follows is a tongue-in-cheek deconstruction of fantasy novels with Westley and Buttercup reconnecting during a series of misadventures involving gentle giant Fezik and the vengeful Spaniard Inigo Montoya.
In a Renaissance-era world, a beautiful young woman named Buttercup lives on a farm in the country of Florin. She delights in verbally abusing the farm hand Westley, whom she addresses as "farm boy", by demanding that he perform chores for her. Westley's response to her demands is always "As you wish." She eventually realizes that what he is really saying is, "I love you." After Buttercup realizes, as well, that she truly and strongly loves him and confesses her mutually strong romantic feelings, Westley leaves to seek his fortune, so they can marry. Buttercup later receives word that the Dread Pirate Roberts, who is notorious for killing all those whose vessels he boards, attacked his ship at sea. Believing Westley dead, Buttercup sinks into abject despair, declaring, "I will never love again." Some time later, she reluctantly agrees to marry Prince Humperdinck, heir to the throne of Florin, due to a law allowing the prince to choose any unmarried woman as his bride, though she makes it clear that she does not love him.
Before the wedding, a trio of outlaws—the Sicilian criminal genius Vizzini, the Spanish fencing master Inigo Montoya, and the enormous and mighty Turkish wrestler Fezzik—kidnap Buttercup. A masked man in black follows them across the sea and up the Cliffs of Insanity, whereupon Vizzini orders Inigo to stop him. Before the man in black reaches the top of the cliff, a flashback of Inigo's past in the novel reveals that he is seeking revenge on a six-fingered man who killed his father. When the man in black arrives, Inigo arranges a fair fight, allowing his opponent to rest before the duel. The man in black wins the duel, but out of respect, he leaves the Spaniard alive. Stunned, Vizzini orders Fezzik to kill the man in black. Another flashback occurs detailing Fezzik's history of reluctant fighting matches since childhood. His conscience compelling him, Fezzik throws a rock as a warning and challenges the man to a wrestling match. The man in black accepts the challenge and chokes Fezzik until the giant blacks out. He then catches up with Vizzini and proposes a battle of wits, guessing which cup of wine is poisoned with iocaine powder. They drink, and the man in black reveals that both cups were poisoned, but he hadpreviously cultivated an immunity to iocaine powder. Vizzini dies.
With Prince Humperdinck's rescue party in hot pursuit, the man in black flees with Buttercup. He taunts Buttercup, claiming that women cannot be trusted and that she must have felt nothing when her true love and sweetheart had died. Enraged, she shoves him into a gorge, yelling, "You can die, too, for all I care!", only to hear him call, "As you wish!" from the bottom of the ravine. She realizes he is none other than her dearly beloved Westley, and follows him down into the gorge, to find him battered but largely unhurt. While traveling through the Fire Swamp to evade Humperdinck's party, Westley tells Buttercup that the Dread Pirate Roberts did attack his ship, but kept him alive after he explained the depths of his love for her. Westley became the Dread Pirate Roberts' valet, and later his friend. Over the course of four years, Westley learned how to fence, fight, and sail. Eventually, Roberts secretly passed his name, captaincy, and ship to Westley, just as his predecessor had done.
After facing many trying ordeals such as Snow Sand and Rodents of Unusual Size, Westley and Buttercup successfully navigate the Fire Swamp, whereupon they are captured by Prince Humperdinck and his cruel six-fingered assistant, Count Tyrone Rugen. Buttercup negotiates for Westley's release and returns with Humperdinck to the palace to await their wedding. Rugen follows Humperdinck's secret instructions to not release Westley, but to take him to the fifth level of his underground hunting arena, the "Zoo of Death". Here, Rugen gravely tortures and weakens Westley with his horribly painful life-sucking invention, The Machine, to obtain first-hand information for completing his definitive book on pain along with appeasing Humperdinck's annoyance that Buttercup does and always will prefer Westley to him.
Meanwhile, Buttercup has several nightmares regarding her marriage to the prince. She expresses her unhappiness to Humperdinck, who proposes a deal wherein he will send out four ships to locate Westley, but if they fail to find him, Buttercup will marry him. The novel reveals that, to start a war with the neighboring country of Guilder, Humperdinck himself had arranged Buttercup's kidnapping and murder, but that he now believes that Buttercup dying on her wedding night will inspire his subjects to war even more effectively.
On the day of the wedding, Inigo meets again with Fezzik, who tells him that Count Rugen is the six-fingered man who killed his father. Knowing that Vizzini is dead, they seek out the man in black hoping that, if he could outsmart Vizzini, his wits will help them plan a successful attack on the castle to find and kill Count Rugen. Buttercup learns that Humperdinck never sent any ships, and taunts him with her enduring love for Westley. Enraged, Humperdinck tortures Westley to death via The Machine at its maximum setting. Westley's death screams echo across the land, drawing Inigo and Fezzik to the Zoo of Death and down through its many dangerous levels. Finding Westley's body, they enlist the help of the King of Florin's former "miracle man", a magician named Miracle Max who was fired by Humperdinck. Max pronounces Westley to be merely "mostly dead", and returns him to life (out of a desire to get back at Humperdinck), though Westley remains partially paralyzed and weak.
Westley devises a successful plan to invade the castle during the wedding, and the commotion caused by this prompts Humperdinck to cut the wedding short. Buttercup decides to commit suicide when she reaches the honeymoon suite. Inigo pursues Rugen through the castle, and, reciting aloud his long-rehearsed oath of vengeance ("Hello. My name is Inigo Montoya. You killed my father. Prepare to die.") throughout the duel, kills him in a sword fight. Westley reaches Buttercup before she commits suicide. Still partially paralyzed, he bluffs his way out of a sword fight with Humperdinck, who shows himself to be a coward. Instead of killing his rival, Westley decides to leave him alive for a long, miserable life with his obvious cowardice as his only companion. The party then rides off into the sunset on four of the prince's purebred white horses which Fezzik had conveniently discovered. The story ends with a series of mishaps and the prince's men closing in, but the author indicates that he believes that the group got away.