The Simpsons Movie (PG)

Time was when people heard the name "Homer" and thought of the Odyssey. Now, they are more likely to think of a yellow-skinned, pot-bellied, beer-swilling cartoon dad who greets life's reversals, big or small, with the resonant exclamation "Doh!"

Homer Simpson, the American Everyman as incorrigible oaf, is also the patriarch of perhaps the country's most famous family – his saintly wife Marge (she of the tottering blue beehive), brattish son Bart, right-on daughter Lisa and silent, impassive infant Maggie. A poll discovered that more than one-fifth of Americans could name all five Simpsons – but then, they have had 18 TV seasons and 400 episodes to learn them.

The only surprising thing about The Simpsons Movie is that it has taken the creators this long to make the transition to the big screen. Did they suspect that their half-hour sitcom format was as good as it could get? At the beginning of the film, there's a cute and characteristically arch gag as Homer derides the feature-length movie of Itchy & Scratchy, the superviolent cat-and-mouse cartoon shorts that are a fixture chez Simpson – why pay to watch a movie of them anyway, he asks, when it's something they get for free on TV?

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It's modest of the film-makers to make such a joke against themselves, but I wonder all the same if the movie does offer anything you can't get from the TV series, aside from length. This is three times as long as an episode, though I can't imagine that even die-hard fans will claim it's three times as funny.

Having said that, it is still three, four, five times funnier and smarter than any other sitcom-based movie, ever, so we shouldn't get picky. The script is credited to no fewer than 11 writers, most of whom have been with the show since its inception, which indicates not only the manpower required to hone and model its excellence but also the consistency of personnel maintaining it. Over the 17 years it has been on air we may have become a little complacent about The Simpsons, but I don't think the scriptwriters have. For this one-off, I wondered if they might trace the family story back to its origins, in the way other franchises such as Batman and Spider-Man have recently done; just imagine the fruit that might be plucked from the Simpson family tree.

Instead, James L Brooks, Matt Groening et al have kept faith with the madcap-surrealist comedy that has been the show's trademark and dreamed up a plot whose note of ecological catastrophe resonates with our own stricken times. The first warning of it emerges during a church service, church being one of those places Homer resents because it's not Moe's bar or his own sofa. "Why can't I talk to the Lord in my own time?" he whines to Marge, a private communion he admits would simply constitute "praying like heck on my deathbed".

During the service, Grampa Simpson astonishes the Springfield faithful with a violent fit that has him writhing on the floor and babbling maniacally. Is this a "senior moment" or is it, as Marge guesses, a garbled prophecy of imminent woe?

Suffice it to say that all the trouble emanates from Homer's sudden and inexplicable adoption of a pet pig – inexplicable because he traditionally prefers the animal sizzling in a pan. Owing to his recklessness, incompetence and greed (the usual), Homer almost causes his family to be lynched by a Springfield mob, though the dark machinations of the US government will surpass even him for irresponsible behaviour.

The film pokes quite a bit of fun at the office of the President, occupied (but not voiced) by one Arnold Schwarzenegger who, policy-wise, isn't too hot on the fine print: "I was elected to lead, not read," he declares. The downside of this is that there's not much villainy left over for the show's resident malefactor Montgomery Burns, one of the finest creations in the cartoon firmament. Perhaps they just didn't want to confuse him with Dick Cheney.

What The Simpsons has always brilliantly managed is a double vision of family life, a way of sending it up and at the same time subtly endorsing it. Homer, ever the unregenerate slob and monster of selfishness, here seemingly alienates even Bart, who senses in Ned Flanders the dad he never had. Yet even as disaster looms the Simpson women provide a counterbalance, tending the flame of moral duty and decency: in the characters of Marge and Lisa, the show consistently proves itself a notably gynophile comedy, the former a model of wifely forbearance, the latter a passionate (and only slightly pious) champion of good causes.

Lisa has more on her plate than usual, suffering the throes of a teenage crush on an Irish boy whose dad's a rock star ("It's not Bono," he insists, though Lisa probably wouldn't mind if it was). And how about this for a moment of wry wisdom – Lisa: "I'm so angry!" Marge: "You're a woman – you can hold on to it forever."

In all honesty, it isn't quite the knockout we hoped for. The last third in particular feels sluggish, at least in comparison with the vivid rat-tat-tat rhythm of gags in the first 20 minutes.

Perhaps it's inevitable that when a show has set the bar so high one will demand something more from a supersize incarnation. Better to think of it as a restatement of classic Simpsons virtues – superb dialogue, inspired slapstick, quickfire wit – rather than reaching for higher peaks of comedy greatness. It's just fine, even if you do have to pay for it.