One of the movies with heady advance buzz on the Croisette this week is not what you’d expect: it’s the studio summer tentpole "X-Men 3: The Last Stand." Far from losing momentum as many other franchises tend to do, this third and last installment in the "X-Men" trilogy is "the best of the three," 20th Century Fox chairman Tom Rothman has insisted. Fox is looking to build global awareness at the 2006 festival.

And this despite the big-budget sequel’s troubled history. Star filmmaker Bryan Singer, who had shepherded the first two blockbusters, abandoned this edition to grab the opportunity to reinvent Warner Bros. Pictures’ rival Superman franchise. Rising director Matthew Vaughn ("Layer Cake") also bailed under the pressure of taking on the massive FX sequel. At which point, on June 6. with the clock ticking toward a May 26 release date, Brett Ratner ("Red Dragon") took over the directing helm.

But the person who steered a steady course through the madness was producer Lauren Shuler Donner, one of the few bona fide star producers in Hollywood who really defines the term. For some 25 years she not only has developed her own material but also closely supervised production of a range of genres, from studio comedies ("She’s the Man," "Dave") and auteur-driven projects (Nora Ephron’s "You’ve Got Mail," Oliver Stone’s "Any Given Sunday") to such big-budget actioners as "Constantine" and "Timeline." It seems she can do just about anything: her upcoming films include Paul Feig’s Christmas comedy "Unaccompanied Minors," for Warners; a screen adaptation of the best-seller "The Secret Life of Bees," starring Dakota Fanning, for Focus Features; and a possible family franchise for Universal Pictures, "Cirque du Freak," based on a series of 12 English children’s books.

But Singer leaving to do "Superman Returns" — which he discussed openly with Shuler Donner and her husband Dick Donner, the director of the 1978 "Superman"— was tough to take. Particularly since he took all his unexpressed ideas for "X-Men 3" with him. "It was a blow," Shuler Donner admits. "He’s my friend, I respect him, and saw us doing all the ’X-Men.’ "

Pushing forward, nailing the ultimate script was the main concern. Shuler Donner, Marvel movie czar Avi Arad and Fox’s production team brought back one of the "X-Men 2" writers, Zak Penn, and teamed him with Simon Kinberg ("Mr. & Mrs. Smith"). "It’s the final chapter of the trilogy," Shuler Donner says. "We set up the dark phoenix story in ’2,’ and we pay it off in ’3.’ This film is about an issue: Before, it was about being a mutant in a world that hated mutants. Now mutants are part of the government. We delve into the issue of a cure, which is the antithesis of the X-Men world. It implies that mutancy is a disease. Should the cure be attacked or accepted?"

After the production replaced Singer with Vaughn, the new director worked closely with the writers. Even though Shuler Donner is still furious with Vaughn for leaving, she confirms Kinberg’s assertion that Vaughn contributed some brainpower to the script. "We cast Kelsey Grammer with Matthew," she admits, "who was fantastic, and Vinnie Jones as Juggernaut. We were lucky to have him."

But Vaughn’s departure threw a monkey wrench into a huge $150 million production that was already stretching its resources to meet its opening date. "Then suddenly, weeks of prep time were lost," she says. "It killed us. It was an extremely desirable release date. There was no way we were going to miss that date. We had to protect it by any means possible." Go off that date and "X-Men 3" would bump into the likes of such other summer releases as Singer’s own "Superman Returns" and "Pirates of the Caribbean: Dead Man’s Chest."

Luckily, Ratner’s next project at Universal was falling apart. The "X-Men" team moved swiftly to snatch him up. "He had handled large action movies successfully," Shuler Donner says. "We knew he could handle depth of character, had an admirable visual style and had a desire to do a comic franchise movie. So it seemed like the right match. He brought volumes of enthusiasm. We were under the gun. With a long, arduous task in front of you, that enthusiasm goes a long way. Our concerns were only about prep time, time, time."

Like any self-respecting A-lister, Ratner wanted his own rewrites, of course. He changed about a third of the script, Shuler Donner estimates. "It’s the best script of the three," she says. "It’s the most focused and complicated and serious. It’s more like ’Spider-Man.’ The audience is smarter and smarter and they get it faster. You can bring heavy themes into a big action movie and get away with it. We all know more and more what to do. And the actors know their characters better. And we bring in new ones to complicate things."

What distinguishes "X-Men" from many studio franchises like "Mission: Impossible" is its rich source material: The film series has been able to draw from 40 years of comic books. That’s what drew Shuler Donner to get involved initially, she says: "I wanted to emulate ’Star Trek’ and do many of them. There were enough stories to do that." After writer-director Joss Whedon wrote about the mutancy cure in his recent "X-Men" comics, that sent the filmmakers back to a similar 10-year-old story line from a Saturday morning "X-Men" cartoon.

This time out, Halle Berry as Storm gets to kick some serious butt — and moved so fast during her wire shots that she had to take Dramamine. "She wanted to fly and fly she does," Shuler Donner says. "The women superheroes are as vital as the males. They’re part of the world: I’d argue that Storm, Jean Grey, Kitty and Rogue are big Marvel action heroines."

From the beginning, the studio and filmmakers favored including many of the younger characters, which include a mix of men and women dominated by feral adult Wolverine (the hirsute Hugh Jackman). "That way high school kids could identify with it," Shuler Donner says. "Their powers come out during puberty."

The other weapon in the "X-Men" arsenal is its legion of fans, who have been nothing if not vocal about their wants and desires for the series. It’s been possible to keep the series fresh by adding new characters from the comics. But which ones to add this time was a subject of some debate. The filmmakers decided to listen to the fans, who had been begging for more of Kitty Pride, plus Juggernaut, Beast and Angel. Gambit, on the other hand, was deemed too similar to "2’s’’ Nightcrawler, Shuler Donner says.

Getting the movie done in time was a feat. Juggling the availability schedules of 14 major cast members, many in demand, was a headache. "Everyone had another movie," the producer recalls. "We had to finish on time. If we went over, we’d cut into postproduction. And if the visual effects weren’t ready, we couldn’t afford one cheesy effect."

The VFX work was massive. The FX team was designing many new characters, most notably Beast and Angel, who boasts a full set of white wings. Always under pressure to outperform previous effects shots, "X-Men 3" has had the luxury of not having to do effects for effects sake, so that when Magneto throws cars at Alcatraz off the Golden Gate Bridge, he’s simply exercising his super-powers. "We always had the mechanism that our characters had powers, so they had to be effects that don’t exist in the real world," Shuler Donner says. "That helped to define us, and helped us not to be excessive or exploitative."

"X-Men" introduces a nifty piece of new FX software that could be much in demand in the future: the rejuvenation effect Lola, which FX master John Bruno helped to perfect, makes actors look younger. It was used for a subplot involving the youthful incarnations of Patrick Stewart and Ian McKellan, making them 20 years younger. "They loved it," Shuler Donner says. "Would that it would work in real life."

With the trilogy complete, Shuler Donner is forging on with such offshoots as "Magneto" and "Wolverine." "X-Men," it seems, is an ever-mutating universe.