Director Denis Villeneuve impressed “Dune” fans with the first half of his adaptation of Frank Herbert’s classic novel in 2021, but “Dune: Part Two” is a real masterpiece of sci-fi filmmaking. What makes Villeneuve’s take on the “Dune” universe so compelling is how faithful it is to Herbert’s original vision — or at least to Villeneuve’s understanding of that vision.

There are definitely things that Villeneuve’s “Dune” movies get wrong from the book, but most of the deviations have a clearly defined purpose. Villeneuve made changes that he thought improved the source material, and his tweaks managed to fix some of his own problems with the novel. One of Villeneuve’s biggest issues with Herbert’s novel is how it handles Lady Jessica (Rebecca Ferguson) in the back half of the story. “She’s still Lady Jessica, the main architect of the story,” Villeneuve told Total Film magazine. “I thought that was a very powerful idea that was not sustained in the book. I made sure that she has the character presence in the second part.”

Villeneuve’s approach to “Dune: Part Two” makes Lady Jessica a more important character, which also sets up her role in the director’s upcoming adaptation of “Dune: Messiah.” While Herbert didn’t know for certain that he’d be writing a sequel to “Dune,” Villeneuve used that advance knowledge to shuffle some major characters around in “Dune: Part Two.”

The ending of “Dune: Part Two” follows the same general path as the ending of Frank Herbert’s “Dune.” Paul Atreides (Timothée Chalamet) leads the Fremen into battle, starting a war that will spread throughout the stars. In the ensuing chaos Baron Harkonnen (Stellan Skarsgård ) is killed and Paul is named Emperor. Those events occur in both versions of the story, but there’s a major player missing from the battle in Denis Villeneuve’s movie.

In the book Paul spends two years living with the Fremen, and in that time his mother, Lady Jessica, gives birth to his sister Alia. The movie hints at Alia’s upcoming role in the story with a brief flash-forward to the future (in which an older Alia is played by Anna Taylor-Joy), but in the book her prescient vision makes her unnervingly adult-like when she’s still a toddler. Alia uses that super intelligence to her advantage and joins the final battle, and the way that Herbert wrote the story, it’s actually Alia — as a child — who kills Baron Harkonnen.

“We were a little leery of that talking toddler, as a distraction in the middle of the film,” Jon Spaihts, co-writer on both of Villeneuve’s “Dune” movies, explained to Inverse. “That’s a difficult thing to execute on film.” Difficult, but not impossible: David Lynch’s 1984 “Dune” adaptation did include little Alia (Alicia Witt) hopping into battle, but after seeing that, even “Dune” purists will probably agree that Villeneuve and company made the right call.

By beefing up Lady Jessica’s role in “Dune: Part Two,” Denis Villeneuve was able to fix one of his personal problems with Frank Herbert’s original novel, while cutting Alia’s character from the second film allowed Villeneuve to streamline the story and avoid making audiences confront the visual of a murderous toddler. Those are big changes, but neither of them were as impactful as the change Villeneuve made to the role Chani (Zendaya) plays in the story.

In the book, Chani is a true believer in Paul Muad’Dib. The two of them still fall in love with each other, but Chani never doubts the religious significance of Paul’s arrival on Arrakis. Villeneuve has said that in the movie, Chani’s skepticism plays an important thematic role. “Dune: Messiah” explores the idea that Paul is not a perfect savior, and Villeneuve wanted to get those ideas flowing early by making Chani a skeptic.

“In the book she’s in the shadow of Paul, but we wanted to give Chani her own agenda here, her own beliefs, her own tale,” Villeneuve told GamesRadar, adding, “She becomes our moral compass. It was therefore very important to finish the movie on Chani as that’s our angle, our take.” Going into the third movie, Chani finds herself in a very different place from her book counterpart, and watching Villeneuve explore that will likely be one of the most fascinating aspects of “Dune: Part Three.”