A grandfather reads a novel to his sick grandson, who initially dismisses the story.

The tale is about Buttercup, a 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." The two fall in love, and Westley leaves to seek his fortune overseas so they can marry. When his ship is attacked by the Dread Pirate Roberts, Westley is presumed dead.

Five years later, Buttercup is forcibly betrothed to Florin's Prince Humperdinck. Before the wedding, she is kidnapped by three outlaws; a small Sicilian man 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. A masked man in black pursues them, as does Prince Humperdinck and his soldiers.

The man in black confronts the outlaws atop the Cliffs of Insanity. He defeats Inigo in a fencing duel and knocks him out, chokes Fezzik into unconsciousness, and tricks Vizzini into drinking a deadly poison. He forcibly flees with Buttercup. Buttercup correctly guesses he is the Dread Pirate Roberts, berates him for killing Westley, and shoves him into a gorge. While tumbling down, he shouts, "As you wish!" Realizing it is Westley, she throws herself into the gorge after him, and they are reunited.

As they make their way through the dangerous Fire Swamp to avoid Humperdinck and his men, Westley explains how "Dread Pirate Roberts" is an inherited title that he assumed when the previous Roberts wished to retire. Having found Buttercup, Westley intends to surrender the title to another. Humperdinck captures the pair after they emerge from the Fire Swamp. Buttercup agrees to return with Humperdinck after he promises to release Westley. Humperdinck 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 Rugen has six fingers on his right hand.

When Buttercup threatens suicide if the wedding happens, Humperdinck falsely promises to find Westley for her. His real plan is to start a war with the neighboring country of Guilder by killing Buttercup and framing Guilder for it. He had secretly hired Vizzini to do this before Westley interfered. Meanwhile, Fezzik becomes part of the brute squad ordered to clear the thieves' forest before the wedding. He finds a drunk Inigo living in the forest, whom he sobers up and tells him about Rugen. Inigo says that he and Fezzik needs Westley's help to storm the castle.