At the ripe old age of 20, cartoon franchise The Simpsons has finally entered the ring. Banish thoughts of donuts and/or anal sex from your head (though both have their part to play in The Simpsons Movie). We're talking about the grand arena known as cinema, where whipper-snapper TV comedy rivals South Park and Borat! have already scored huge victories.

According to its critics, Matt Groening's zeitgeisty, irreverent show has long succumbed to middle-age spread. Does the much-hyped feature film deliver a knock-out punch? Or does it fall flat on its yellow belly before the bell has even rung? The truth lies somewhere inbetween.

The plot has Homer inadvertently causing an ecological disaster in his home town of Springfield. As a result, the Environment Protection Agency decide to put a lid on the place - literally, a dome descends. Managing to evade an angry mob, Homer and his brood take refuge in Alaska, where they realise the fellow in charge of the EPA has a new, even more terrible fate in store for their friends and neighbours. Can they save the town? More importantly, do they want to?

Homer, initially at least, is in grand form. Much to his family's horror he has fallen in love with a pig (aka Spiderpig aka Harry Plopper). While watching TV, the pair see a man and beast brush lips. Homer gulps, turns to the swine and thinks, "Maybe we should kiss, just to break the tension." Which is funny, right? At this point - sad sack that I am - I scribbled "Phew!" in my notes.

The film's non-villainous villain, EPA chief Russ Cargill (voiced by the brilliant Albert Brooks), is also intriguing. He explains to the country's President, Arnold Schwarzenegger, that he is a rich man who has taken on this job because: "I wanted to give something back. Though not money."

He later goes into a surreal rant involving the words tough and soft, prompting a minion to observe, "Sir, I'm afraid you have gone mad with power." To which Cargill replies, with great astuteness, "No one goes mad without power! It's boring. No one listens to you."

Trouble is, this is as good as the character - or the doomsday scenario - gets. Cargill proves neither scary nor charming enough to hold our attention, while the family's emotional crises feel increasingly routine. Whether out of nerves or complacency, the scriptwriters have rehashed many of the TV shows' plots, minus the zest.

The emphasis is on slapstick, and even Lisa Simpson (the character closest to the hearts and minds of the show's scriptwriters) is played for broader laughs. There are probably one or two people in the world who have never watched an episode of The Simpsons. The film, at its worst, panders to them.

The TV show's scriptwriters, of course (many of whom were recruited by Matt Groening for this film), have always known how to pre-empt criticism. A regular character, Comic Book Guy, always greets new ventures with the words "Worst ever ..." Anyone who carps about this film risks striking the same, dour note.

So let's look on the bright side. There's an inspired, brazenly bizarre sequence where Disney-ish animals undress Marge and Homer in Alaska. The animals, horrified but enthralled, then watch the couple having sex. Later, Bart puts a bra on his head and pretends to be Mickey Mouse: "I am the mascot of an evil corporation!" The writing team obviously have a love-hate relationship with Walt, the genius who put animation on the map, and sold its soul for a few theme parks.

Similarly awkward, but interesting, are the sub-titles advertising a TV show on Fox (The Simpsons' parent company). The Simpsons Movie is brought to you by an evil corporation. Watching the writers try to make sense of that is entertainment all in itself.