Bursting with action, X2: X-Men United is a double rarity: a sequel that improves on the original, and a big-budget blockbuster that actually has something to say. With complex characters, a fast-paced plot and dazzling special effects, the film stakes an early claim in the summer box-office competition.

X2 picks up right where X-Men left off. Professor Charles Xavier (Patrick Stewart) is still fighting government attempts to reign in mutants. Wolverine (Hugh Jackman) has gone to frigid Alkali Lake to find out more about his past. Telepathist Jean Grey (Famke Janssen) wonders if she should have chosen Cyclops (James Marsden) over Wolverine. Rogue (Anna Paquin) has a new boyfriend, Bobby (Shawn Ashmore), a mutant nicknamed Iceman for his freezing powers. And Magneto (Ian McKellen) is still being held prisoner is a specially designed plastic cell.

When Kurt Wagner (Alan Cumming), a mutant known as Nightcrawler, attacks the White House, almost pinning the President to his own desk in the Oval Office, the evil Col. William Stryker (Brian Cox) sees an opportunity to rid the planet of mutants once and for all. His first target: Xavier's prep school in upstate New York. Stryker's nighttime attack puts Wolverine on the run with Rogue and her friends.

While Jean and Storm (Halle Berry) track down Nightcrawler, Mystique (Rebecca Romijn-Stamos) engineers Magneto's escape. She's found clues that Stryker is building a new version of Xavier's Cerebro supercomputer. To his amusement, Magneto must team up with his arch-rival Xavier to defeat Stryker. They meet in an underground compound, where Wolverine is pitted against Deathstrike (Kelly Hu), while Jean finds herself fighting Cyclops.

Director Bryan Singer adopts a bold, graphic style, using skewed angles and tight close-ups to approximate the look of comic books. His approach is especially effective in the bravura opening, as Nightcrawler decimates White House Secret Service agents, or when Storm and Jean try to dodge heat-seeking missiles in the X-Men jet. The action throughout is a seamless synthesis of stuntwork and state-of-the-art special effects that leaves viewers giddy with excitement.

Singer is adept with intimate details as well. 'Don't show off,' Xavier warns the mutants at one point, and it's advice the cast takes to heart. The shrewd screenplay lets the actors remain true to their characters' histories, while punctuating almost every scene with troubling ambiguities. Singer also honors the comic book's left-wing tone. X2 raises pertinent questions about personal freedom and the dangers of an unchecked military, a risky move in today's political climate. Judging by how the camera favors Mystique's breasts when Magneto has a chunk of exposition to deliver, the filmmakers haven't lost track of their core audience either. In fact, there are very few mistakes in X2, leaving everything in place for the next sequel.

--Daniel Eagan