The film is an enactment of a book that a grandfather reads to his sick grandson, who initially dismisses the story.
The tale is about Buttercup, a beautiful young woman living on a farm in the fictional kingdom of Florin. Whenever she tells farmhand Westley to do something, he always complies, saying, "As you wish." She eventually realizes that he loves her and she loves him. Westley leaves to seek his fortune overseas so they can marry. When his ship is attacked by the Dread Pirate Roberts, who is infamous for never leaving survivors, Westley is presumed dead.
Five years later, Buttercup is forcibly betrothed to Prince Humperdinck, the heir to the throne of Florin. Before the wedding, she is kidnapped by three outlaws, a short Sicilian named Vizzini, a giant from Greenland named Fezzik, and a Spanish fencing master named Inigo Montoya, who seeks revenge against a six-fingered man who murdered his father. Prince Humperdinck, his soldiers, and also a masked man clad in black, pursue the outlaws.
The man in black confronts the outlaws atop the Cliffs of Insanity. He defeats Inigo in a duel and knocks him out, chokes Fezzik into unconsciousness, and tricks Vizzini into drinking poison, killing him. He takes Buttercup prisoner and they flee, stopping near a gorge. Buttercup correctly guesses he is the Dread Pirate Roberts and berates him for killing Westley. Seeing Humperdinck and his men approaching, Buttercup shoves Roberts down a hill, wishing death upon him. While tumbling down, he shouts, "As you wish!" Realizing it is Westley, she throws herself into the gorge after him, and they are reunited.
Westley explains how "Dread Pirate Roberts" is a title that is passed on to others; he took it when the previous Roberts wanted to retire. Having found Buttercup, Westley intends to surrender the title to another. Humperdinck captures the pair after they emerge from the dangerous Fire Swamp. Buttercup agrees to return with Humperdinck after he promises to release Westley. He then secretly orders his sadistic vizier, Count Rugen, to take Westley to his torture chamber, the Pit of Despair. Before being knocked out, Westley notices that Count Rugen has six fingers on his right hand and realizes that he is the man who killed Inigo's father.
Humperdinck falsely promises Buttercup he will search for Westley. His real plan is to start a war with the neighboring country of Guilder by killing Buttercup and framing Guilder for her death. Meanwhile, Inigo and Fezzik are reunited after Humperdinck orders thieves to be arrested in the forest in which they are staying. Fezzik tells Inigo about Rugen. Inigo realizes he needs Westley's help to storm the castle.
Buttercup accuses Humperdinck of failing to search for Westley. Enraged, Humperdinck imprisons Buttercup and tortures Westley to death. Inigo and Fezzik, who have heard and followed Westley's wails through the forest, find his body and bring him to Miracle Max, a folk healer. Max revives the "mostly dead" Westley, though he is severely weakened.
As Westley, Inigo and Fezzik storm the castle, Humperdinck panics and orders the in-progress wedding ceremony to be shortened. Inigo finds and kills Rugen in a duel after repeatedly taunting him for killing his father. Westley locates Buttercup, who is about to commit suicide, believing she is married to Humperdinck. Westley assures her the marriage is invalid because she never completed her wedding vows. Humperdinck finds them and attempts to kill Westley in his defenseless state, but Westley wills himself to his feet and intimidates the prince into surrender. Buttercup and Westley leave Humperdinck tied to a chair as they flee the castle. After killing Rugen, Inigo is unsure of what to do with his life, and Westley offers him the Dread Pirate Roberts title. Fezzik has procured four horses, and he, Westley, Buttercup, and Inigo escape. Westley and Buttercup, safely reunited, share a passionate kiss.
Back in his bedroom, the boy eagerly asks his grandfather to read him the story again the next day, to which his grandfather replies, "As you wish.”
