Grade: A/B

It takes a big movie to warrant starting the “summer” movie season on the second day of May. “X2: X-Men United” is the rare sequel that can carry such weight.

Critics of the first X-Men film claimed it was too short, too expository and inadequately action-packed. “X2” is longer and filled with action, and, if anything, slightly lacking in thorough explanations.

But most films expect us to assume a lot. The demands of “X2” on its audience are easily met, and we are rewarded with what might be the best franchise-building sequel since “The Empire Strikes Back.”

If the first X-Men film faced a huge challenge of pleasing the sizable fan-base of the now-40-year-old comic series while attracting the average moviegoer, “X2” faces the additional difficulty of living up to and building on the first movie while attempting to stand alone as a believable sci-fi, action-comic-book flick. Apart from a few necessary references to the first film, “X2” holds up well on all levels.

With a vast universe of odd and intriguing characters from which to draw, the architects of the franchise introduce new mutants with deliberate care. Minor characters from the first film, such as “Iceman” Bobby Drake (Shawn Ashmore, reprising his role), are developed further, and new ones like metal-skinned “Colossus” (newcomer Daniel Cudmore) are given introductions for likely larger roles in sequels to come.

Moreover, other fan-favorites are thrown whole-hog into the mix. The film opens with an engaging action-sequence featuring blue-skinned, prehensile-tailed, teleporting mutant “Nightcrawler” (Alan Cumming, “Titus”) staging a one-mutant assault on the White House.

Elements of the character vary from the comic to the likely annoyance of some fans — e.g., his Catholicism is presented as his major personality trait, and the “fuzzy blue elf,” while unquestionably blue and somewhat elfin, is not fuzzy.

Nitpicking aside, he and all the other characters are as convincing as super-powered mutants can be.

All the principals — those who survived the first film, anyway — are back and more thoroughly fleshed-out. Only “Cyclops” (James Marsden, “Disturbing Behavior”) remains underdeveloped, forfeiting screen-time to his better-known co-stars.

The story serves as an adequate but not-too-complex framework for the action and character development. It revolves around high-ranking government official William Stryker (Brain Cox, “The Rookie”) waging a sanctioned vendetta war against mutant-kind. The plot thickens as his past is revealed to intertwine with those of Wolverine (Hugh Jackman, “Swordfish”), and Professor Charles Xavier (Patrick Stewart, “Star Trek: Nemesis”) and his Westchester, N.Y.,-based mutant-training academy.

It is mildly disappointing that “X2″‘s real highlight comes rather early in the film, as the mutants must defend the school against armed invasion. The scene should satisfy fans who felt Wolverine’s ass-kicking skills were sold short in “X1,” but it renders his battle with “Deathstrike” (Kelly Hu, “The Scorpion King”) less climactic than it might have been. This is also owing to the limited development given to Hu’s character.

Beyond these minor faults, there are only a couple chinks in “X2’s” armor of believability.

The sound effects become excessive and obvious — even beyond overwrought, industry-standard fight sounds. (When will filmmakers learn that punching someone rarely, if ever, sounds like whacking a raw side of beef with a cricket bat?) What overdoes it in “X2” is the deafening sound of knuckle-cracking. Such a small element does a great deal to dispel the film’s overall credibility.

Of greater concern is the abbreviated passage of time. Maybe in later sequels we’ll be introduced to the mutant whose special ability is the amazingly rapid assembly of incredibly sophisticated technical apparati. Meanwhile, we’ll just have to accept the necessity of cutting some corners to make a superhero film flow and maintain excitement.

When all is said and done, the shortcomings should not overturn the success of the film. Director Bryan Singer (“The Usual Suspects”) has planned ahead, and “X2” feels like the second step on a long, lucrative road.