Jaunty, jokey, and absolutely bursting with visual energy, "Monsters vs Aliens" is unquestionably a treat. If it had a more substantial story, it could be a classic. As it stands, it's a shrewd, fast-paced confection that takes full advantage of its 3D presentation (you should definitely see it that way) and offers plenty for kids and adults to laugh at.

The plot is impossibly simple. When an alien being named Gallaxhar (voice of Rainn Wilson) threatens Earth, the U.S. government (why do WE always have to be the ones to respond?!) unleashes its secret weapon: a squad of monsters that the military has been hiding for decades. The point has always been to keep these monsters from harming the general populace; now there's no alternative but to set 'em loose and let 'em fight the aliens.

Naturally, these are pretty tame monsters anyway, fearsome only in their appearance, genetic makeup, and/or size. The latest addition to the team is Susan Murphy (Reese Witherspoon), who was hit by a meteor on her wedding day and subsequently grew to some 50 feet in height. The monster division of the U.S. Army, led by Gen. W.R. Monger (Kiefer Sutherland), captured her, dubbed her Ginormica (all monsters need cool names), and locked her up with her fellow freaks.

These include Dr. Cockroach (Hugh Laurie), a mad scientist who's now part bug; Missing Link (Will Arnett), a mammalian-reptilian-human hybrid who fancies himself something of a ladies' man; B.O.B. (Seth Rogen), a cheerful gelatinous blob who has no brain and has found he doesn't need one; and Insectosaurus, a 300-foot furry bug with a Godzilla roar and no harmful agenda whatsoever.

Despite the apocalyptic trappings -- Earth is threatened! There are monsters afoot! -- the film is completely focused on adventure and comedy, not scares. (The villain is voiced by Dwight from "The Office," for crying out loud.) An action scene set on the Golden Gate Bridge is thrillingly rendered, with many people imperiled but no one hurt. That's an important distinction in a kids' movie. These are the fun kind of aliens and monsters, not the dangerous kind.

The screenplay is credited to five people whose combined writing credentials include "The Simpsons," "King of the Hill," "The Larry Sanders Show," and "Kung Fu Panda," and the directors, Rob Letterman and Conrad Vernon, made "Shark Tale" and "Shrek 2," respectively. I don't know why five people were needed to tell a story this slight, but the script is fairly well packed with subtle references to everything from "Dr. Strangelove" to "E.T." There are also clever running gags that spoof sci-fi cliches (including an obtrusive identity-confirmation device that goes beyond merely scanning your retina), and nerdy jokes like the exclamation "By Hawking's chair!"

The lively voice work (including Stephen Colbert as an idiot U.S. president who looks like Gregory Peck in "To Kill a Mockingbird") is excellent all around. What the film lacks that keeps it from being particularly memorable is subtext. Why not explore the fact that these "monsters" are nice creatures who only LOOK terrible? Why not give them goals greater than simply gaining freedom from the government holding facility? Only Susan/Ginormica has a character arc, as she gains confidence to be her own person and not live in the shadow of her fiance (a preening weatherman voiced by Paul Rudd). Surely there's room for the supporting characters to grow, too, and not just in size.

Grade: B

Rated PG, a few crude jokes

1 hr., 34 min.

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