The new computer-graphic-animated film “Horton Hears a Who!,” like Dr. Seuss’s beloved children’s book, on which it is based, begins its story on the 15th of May in the Jungle of Nool. Almost everyone born after the book was published in 1954 knows what happens next: “In the heat of the day, in the cool of the pool, he was splashing, enjoying the jungle’s great joys, when Horton the elephant heard a small noise.” The rest of the story is spun from the reverberation of that small noise, which comes from a speck of dust that contains a tiny civilization. From there, movie and source material vary a bit, as they are wont to do.

The film’s lovely opening sequence follows the speck of dust from its safe resting spot to a more precarious existence near Horton’s lazy bath. The “camera” swoops through millions of pixels’ worth of lush jungle greenery and — true to Dr. Seuss’s surreal palette — pinkery and bluery and orangery, too. This sweet sequence is the calm before the storm introduced by Jim Carrey, who provides the voice of Horton and was obviously the inspiration for many of the elephant’s contortionist moves.

It’s unfortunate that, by nature, the character’s trunk hides his mouth; it makes him less naturally expressive than he might have been and perhaps amps up the filmmakers’ instinct for animated exaggeration. The book’s Horton is less manic, but manic times call for manic movies (this one’s produced by Fox’s Blue Sky Studios). “Horton” bears more resemblance to “The Grinch Who Stole Christmas,” the garish 2000 Dr. Seuss movie starring Mr. Carrey, than to the quiet, animated versions of each that preceded them.

Conflict comes in the person of a “sour kangaroo,” voiced by Carol Burnett. The kangaroo doesn’t believe in belief. She certainly doesn’t believe Horton’s proclamation that “a person’s a person, no matter how small.” When Horton tells the young jungle children about the speck and the need to protect it, they question authority. That, she insists, leads to anarchy, so Horton must be stopped, “for the children.” The children are a motley group that includes some vaguely animal-like creatures and some wholly invented Seuss-esque fluff-balls. They’re delighted by Horton’s imagination, but he still can’t convince anyone to take his quest seriously.

Horton is in some ways the reverse of Mr. Carrey’s role in the 1998 film “The Truman Show,” in which the actor played a man who was the only one who didn’t realize the smallness of his own world. In “Horton,” he’s in charge of protecting such a world. The speck of dust that Horton devotes himself to saving is perched precariously on a pink clover and populated by the microscopic Whos of Whoville. That bustling town is led by a mayor (Steve Carell), who is often bullied by his own city council, a group that imperiously insists that “nothing has ever gone wrong in Whoville and nothing ever will.” The Whos don’t know they’re relatively teensy, and they certainly don’t believe in Horton, “the giant elephant in the sky.” The mayor is a true believer, however, so he and Horton conspire to have the speck transmitted to a safe spot on a nearby mountaintop.

That journey is arduous, of course. Horton crosses a mysterious landscape that includes a bamboo forest, a cracked-ground desert, icy mountaintops, and a gorgeous field of clover. Meanwhile, the sour kangaroo vows revenge and enlists the help of a vulture named Vlad (in the book, he’s a “black-bottomed eagle”). Later she assembles an angry mob to help destroy the speck, and its attendant speculation, once and for all. The Whos, jostled by all this bumpy traveling, must make their collective presence known by raising their voices as one.

Vlad is voiced by Will Arnett, an actor who voiced characters in “Ratatouille” and “Ice Age,” along with many commercials. His is a deep, expressive voice that belongs in cartoons. The same goes for the venerable CBS anchor Charles Osgood, who narrates the film with snippets of sonorous rhyme. But the movie is larded with celebrity voices that are not so essential. Amy Poehler, Mr. Arnett’s very funny wife, is wasted in the role of the mayor’s supportive spouse. Seth Rogen plays Morton, a tiny blue creature who’s an ally to Horton, though he doesn’t quite believe the elephant’s story. Morton, perpetually exasperated, sounds exactly like Mr. Rogen’s perpetually exasperated character in last year’s “Knocked Up.” Isla Fisher, Jonah Hill, and Jaime Pressly all contribute forgettable performances.

In the end, “Horton” feels airless in a way that Dr. Seuss’s book never does. The imagery, while technically impressive, isn’t as adventurous, and the script doesn’t have as much fun with words and ideas. Dr. Seuss’s McCarthy-esque Wickershams, for example, are just mean monkeys here, and the book’s flickers of Buddhist and Judeo-Christian philosophy go unexplored. But, luckily for the filmmakers, their audience of children lives by a different doctrine: A movie’s a movie, no matter how blah.

Ms. Graham is an editor at Domino magazine.