Maybe there's a good reason they waited 18 years to make "The Simpsons Movie."

Few television comedies have successfully made the transition to the big screen, and with the exception of "The Flintstones," none has done it with the original cast intact. "The Simpsons Movie" attempts to do both things, and while the result isn't hideous, it also isn't great. It's funny, but it's not funny.

The TV show from which this long-awaited adaptation is drawn is a series of exquisite miniatures, an accumulation of small pleasures played out over 18 seasons. There is no single touchstone episode that fans of the show return to again and again, quoting hallowed lines of dialogue from memory. The only thing epic about it is the size of Homer's caboose.

If ever a situation comedy were perfectly proportioned for television, it's "The Simpsons." But because everything is being done on a grander scale for "The Simpsons Movie," TV-sized laughs tend to rattle around and die in the Dolby-enhanced echo chamber of the multiplex. It's only fair to expect the humor to be more panoramic on the big screen, too. But it isn't.

In fact, the sweep of the production seems to work against it most of the time. "The Simpsons Movie" often is just as funny as the TV show, but at four times the normal length, and outside its customary jewel box setting, sharply observed moments are overwhelmed by the need to pump up the volume and keep things moving along.

So when Bart (voiced by Nancy Cartwright) jumps into a sinkhole yelling "Geronimo!" and Lisa (Yeardley Smith) follows with a more culturally astute cry of "Sacagawea!" the laugh vanishes down the hole with her.

The writers, who come breathtakingly close to outnumbering the animators in the closing credits, have lost none of their gift for self-mockery. Following the "Itchy & Scratchy" cartoon that opens the movie, Homer (Dan Castellaneta) quickly reminds us that they're in on the joke.

"I can't believe we're paying to see something we get on TV for free," he bellows. "If you ask me, everybody in this theater is a big sucker." Then, pointing directly into the camera, he adds, "Especially you!"

Fortunately, this joke arrives at the beginning, when we can all still laugh at ourselves for doing exactly what he says we're doing. By the end of the movie, it's a lot less funny. Direct references to the TV show are fairly frequent, and while at least one is brilliant, others fall flat. That happens a lot in this slightly over-inflated cartoon balloon.

Small, life-sized events, such as Lisa falling for the cute new Irish boy (Maile Flanagan) in town, might serve as the basis for an entire episode of the television series. But here they become part of her character's "story arc," and along with the arcs of Bart, Homer and Marge (Julie Kavner), each forms a rib in the construction of the gigantic zeppelin that carries the plot.

Word of advice: When you're building a comedy that's supposed to be lighter than air, it's probably not a good idea to hang it from a story shaped like the Hindenburg.

These plot contrivances involve Homer falling in love with a pig (a running joke that goes nowhere), the pig contaminating Springfield's drinking water, the Simpsons leaving town in disgrace, then returning for Homer's shot at redemption.

The movie isn't a disaster, just a disappointment. Fans of characters such as Montgomery Burns and Apu are likely to be dismayed by their fleeting appearances. And while it's nice to know that if you did indeed eat Bart's shorts, you would find him to be all boy, seeing him fall under the spell of Ned Flanders (Harry Shearer) was almost more than I could bear.

I suspect that anyone unfamiliar with the TV show would be utterly baffled by this movie.

It does look better than the TV show, with a newfound dimensionality to action sequences, such as Bart's naked skateboard ride through downtown Springfield. But with their yellow skin, Marge's blue hair and Homer's dirigible-sized gut, the Simpsons are not the Incredibles. And looks aren't supposed to count.

`The Simpsons Movie'** 1/2

Mercury News Rated PG-13 (irreverent humor throughout)

Voice castDan Castellaneta, Julie Kavner, Nancy Cartwright, Yeardley Smith, Hank Azaria, Harry Shearer, Albert Brooks

Director David Silverman

WritersJames L. Brooks, Matt Groening, Al Jean, Ian Maxtone-Graham, George Meyer, David Mirkin, Mike Reiss, Mike Scully, Matt Selman, John Swartzwelder, Jon Vitti.

Running time 1 hour, 26 minutes