Darkness falls, and Inigo, Fezzik, and Westley prepare to carry out their plan. In the castle’s chapel, the wedding begins, and a clergyman with a considerable speech impediment begins to officiate. As he talks, the wedding guests hear a commotion outside, where Inigo, with Westley on his back, wheels Fezzik forward in the barrow and under the holocaust cloak, terrifying the guards. Fezzik bellows that he’s the Dread Pirate Roberts here for their souls, and Inigo lights the cloak on fire, causing the men to scatter and flee in terror. Back inside, Humperdinck sends Rugen off with some men to deal with the problem, and has the clergyman skip to the end of the officiation. Buttercup gloats that Westley is here to save her. Outside, only Yellin has stood his ground, and Fezzik stops him from lowering the gate. Humperdinck tells Buttercup that he killed Westley himself, but Buttercup doesn’t believe him, noting the fear in his eyes. Inigo threatens to have Fezzik tear Yellin’s arms off if he doesn’t surrender the door’s key, which he promptly does. Humperdinck has the clergyman say “man and wife,” concluding the ceremony, and tells the king to escort Buttercup quickly to her room.
Inigo, Fezzik, and a still mostly paralyzed Westley search the castle halls, coming upon Rugen and his men. Rugen orders his men to kill Inigo and Fezzik and take Westley for questioning, but Inigo defeats them one by one until only Rugen remains. Inigo utters the phrase he’s been waiting 20 years to say: “Hello, my name is Inigo Montoya. You killed my father. Prepare to die.” However, Rugen immediately flees down the hall and through a locked door. Inigo gives chase and calls for Fezzik to break the door down. Fezzik leaves Westley temporarily hanging on a suit of armor to help.
The king and queen escort Buttercup to her room. She pauses to give the king a kiss on the cheek, thanking him for being so kind to her and saying that she plans to kill herself. The senile king celebrates receiving the kiss and doesn’t seem to hear her suicidal threat.
Fezzik returns to where he left Westley to find him gone. Meanwhile, Inigo chases Rugen through the castle. Rugen descends a staircase and waits; when Inigo comes down, he throws a knife into his stomach. Inigo apologizes aloud to his father for failing him. Rugen recognizes him as the former 11-year-old that he bested 20 years before.
Buttercup goes to her room and takes a dagger from its case. She prepares to stab herself, but Westley makes himself known laying on her bed, elating her. She embraces him, but he can’t yet hold her because of the lingering paralysis.
Inigo slowly pulls the knife from his bleeding stomach and gets to his feet, clearly suffering. Rugen attempts to strike him through with his sword, but Inigo weakly deflects it with a swing of his own. Rugen tries again, and again Inigo retaliates. The latter begins to move forward, repeating over and over again his prepared phrase: “Hello, my name is Inigo Montoya. You killed my father. Prepare to die.” Rugen tries and fails to best him, but Inigo regains his strength, fighting harder and quicker and eventually shouting the phrase, before gaining the upper hand and finally cornering Rugen. He slices his left cheek as Rugen once sliced his, and asks for money. Rugen promises it to him. He slices his right cheek, asking for power. Rugen again agrees. Inigo asks for anything he wants, and Rugen promises it to him. Saying he wishes only for his father back, Inigo strikes him through the stomach, killing him. He then runs from the room.
Buttercup apologizes to Westley for getting married, saying she had no choice. Westley clarifies that she didn’t actually say, “I do,” and therefore the marriage is null. Prince Humperdinck appears in the doorway and says he can fix that technicality, but first challenges Westley to a fight to the death. Westley instead promises to horribly disfigure him and condemn him to a life of anguish as a mutilated freak. Humperdinck thinks he’s bluffing, but when Westley stands, Humperdinck drops his weapon. Buttercup ties him to a chair as Inigo finds them. Westley collapses, showing he was feigning having regained his strength. He decides to spare Humperdinck, sentencing him to a life alone with his cowardice. They hear Fezzik calling to them, and find him outside the window with four white horses to take them away to safety. Buttercup jumps down into Fezzik’s arms. Before following her, Inigo explains to Westley that his whole life has been focused on getting revenge on Rugen, and now that he has gotten it, he doesn’t know what to do. Westley suggests he become the next Dread Pirate Roberts, and then jumps out the window after Buttercup. Inigo appears pleased at the idea. The four of them mount their horses and ride off. Westley and Buttercup are about to celebrate their victory with a kiss, when—
The grandfather cuts off the story, remembering that the boy doesn’t like the kissing parts. The boy protests, saying he doesn’t mind them so much anymore. The grandfather then finishes the book, saying that Westley and Buttercup’s kiss put all of history’s previously most passionate kisses to shame. He then advises the boy to go to sleep and dons his hat and coat. As he exits, the boy asks his grandfather if he’ll come over tomorrow to read the story to him again. The grandfather replies happily, “as you wish,” turns out the light, and exits.
It’s in section 5 that we begin to experience simultaneous happenings for the first time. The parallel scenes of the men breaking into the castle as the wedding happens inside raises the stakes for the audience and provides them a bit of high-energy action for the first time. The decline of the overtly parodic story elements has come; we’ve built a legitimate story through making fun of other ones, and now things come to a boil with less comedy mixed in.
Inigo’s final vengeance against Count Rugen marks one of the most serious and impactful moments in the film. This is the part where the jokes and parody elements fade away, revealing the passionate heart of the film underneath them: through silly antics and constant wit, we have become more invested in these characters and their story than any dramatic build up could have allowed for. And we feel Inigo’s redemption as he does, illustrated by the parallel wounds that he inflicts on Rugen that Rugen inflicted on him: two slices on the cheeks, two blows to the shoulders, and finally the strike through the stomach. As Rugen made him suffer, he now makes Rugen do the same, but brings it one step further with a fatal blow for which we cheer. One could even argue that this is the true climax of the film, more so than when Westley bluffs his way into getting Humperdinck to surrender, because it brings us to the peak of our emotional investment in the story, even though it’s technically tying up the film’s secondary plot line.
Related to the above, Westley’s choice not to exact revenge on Humperdinck once he surrenders is a turning point in the film in which revenge is no longer a key player. For most of the story, revenge has been a common theme, fueling Inigo’s quest to find Rugen, Humperdinck's decision to (mostly) kill Westley, etc. After all, these characters are swashbucklers, men of action; you wrong them, they wrong you. Westley, however, has achieved what he wanted—to return to Florin and save his true love. With that accomplished, he removes revenge from the playing field in favor of mercy.
This mercy is in fact a common character trait for Westley, looking back through the film; though he had every right to kill Inigo by the rules of their duel, he chose only to knock him unconscious so that Inigo couldn’t follow him; with Fezzik, he had essentially the same goal; and though his battle with Vizzini resulted in the death of the latter, he first attempted to discuss a peaceable solution with him before Vizzini refused and threatened to kill Buttercup, thus forcing Westley’s hand. Westley’s merciful nature not only makes him a hero worth rooting for, but also ironically saves his life, as his choice to spare Inigo and Fezzik allows them to later bring him back to life with the help of Max and Valerie.
Finally, The Princess Bride’s framing story, having opened the film, now closes it as well. We see how the grandson has developed throughout the story to enjoy all its elements, even the “kissing parts,” a self-proclaimed testament to the heart of the story. The grandfather leaves, having told his tale, but the boy first asks that he come back to read it again the next day, a comment not only on the likability of the story, but on our unquenchable need for more of them; as the boy hopes for another one tomorrow, so too will the viewer—perhaps another movie, perhaps a book, maybe even an uplifting news story on the internet—all in testament to the human need for stories, especially ones that provide us lessons, entertainment, and happy endings. And of course, the grandfather replies with Westley’s catchphrase, “As you wish,” which he himself narrated earlier, and which we know means something else entirely: “I love you.”