Comic book-derived superhero movies are often wildly variable in quality. But the "X-Men" series, based on Stan Lee's Marvel Comics pop classic, has had wider swings than most. The first movie, back in 2000, was a gaudy spectacle with -- for me -- a simple-minded, sometimes roaringly vacuous script. The second installment, "X2: X-Men United," with the same characters and plot devices, was a majestic improvement, emotional and epic where the first one was gaseous and predictable.

Disappointingly, "X-Men: The Last Stand" slides back between the first two episodes. It's not stuporous, and it's not super. It completes, in suitably spectacular fashion but without enough imagination, feeling and verve, the trilogy of movie epics based on Marvel's series about a group of angst-ridden mutant supermen and superwomen.

Superficially, the elements are all there. The movie's main premise is a scientific "cure" for mutancy that sets up a war between Dr. Charles Xavier's (Patrick Stewart) good mutants and Magneto's (Ian McKellen) bad ones, and it brings back most of the core cast. Topping them: eagle-faced Stewart as teacher-mentor Xavier and his followers, including Hugh Jackman's angry Eastwood-ish Wolverine and Halle Berry's weather-manipulating beauty Storm.

Then there are the rebel mutants led by McKellen's Magneto, plus the key character whose sympathies we're not quite sure of: Famke Janssen as reborn Jean Grey (a.k.a. "Phoenix"), who apparently survived the devastating climax of "X2," but may have changed in ways that can't be foreseen (unless you've read the comics).

"Last Stand" adds some other hitherto-ignored characters from the comics -- notably big, bad Juggernaut (Vinnie Jones) and the blue-furred, crusty, lovable Beast, a.k.a. Hank McCoy, played by Kelsey Grammer, who manages to smirk through the latex. (Beast is the U.S. secretary of mutant affairs.)

Also, as you'd expect, there's a feast of CGI thrills, including a technically amazing last big fight, where we see the bad mutants rip apart the Golden Gate Bridge and bend it toward Alcatraz for the final battle.

The stimulus for the battle is that old comic standby, the incredible scientific breakthrough, here financed by billionaire Warren Worthington II (Michael Murphy). Distressed at the pain of his mutant son, Warren III or "Angel" (Ben Foster), who sports two huge white wings on his back, Warren II's empire has developed a "cure" that turns the super-freaks into normal humans. It's an instant process, administered by handguns filled with hypos.

To Magneto and his radical mutants, the cure is an abomination; analogous to a serum that "cures" blackness or homosexuality. These guys want to keep their wild talents, such as Jean's telekinesis and the shape-changing of Rebecca Romijn's outrageously undressed, blue-bodied Mystique (the most unforgettable sight in all three movies). So they fight Xavier's group, who try to stop the warfare and bring everyone together.

Anyway, though the premise is promising and the execution prodigious, the overall effect is never as scintillating as "X2's."

Why would anyone who can fly over San Francisco Bay, or, like Storm, bring on cloudbursts and dance with the whirlwinds, want to be normal? You can feel the pain of Anna Paquin's Rogue, whose power-draining gift messes up her love life so much. You can almost dig Magneto's arguments about the "cure" getting tossed -- if not approve of his methods, which amount to mass carnage.

Somehow, despite the action, it all gets remote and mushy, plopping back between the sometimes-obnoxious first film and the sometimes-sublime second one.

For some, the slide may be because of the change of director. Bryan Singer ("The Usual Suspects"), who showed a marked sensitivity to deviance and outsiders, helmed the first two "X-Men." His replacement, Brett Ratner ("Rush Hour," "Red Dragon"), is a gifted but sometimes almost annoyingly competent high-tech craftsman. And though he's not lacking in emotion, he doesn't generate the level of feeling Singer did in "X2."

Is it possible Ratner doesn't feel the alienation as deeply? More likely, I think, the shallowness of this script does him and the movie in. It's another formula job cooked up by current hotshot Simon Kinberg (of that vacuous hit "Mr. and Mrs. Smith") and the comparative veteran Zak Penn ("X2," "The Last Action Hero.")

Supposedly this is the last of the "X-Men" movies, though several spinoffs (including a "Wolverine" movie) are in the works. But if "X2" excited you, this one brings you down to earth, not with a crash but a snicker. "X-Men: The Last Stand" is spectacular but alienating: a high-flying high-tech product that impresses but fails to move you.