The Invention of Lying, Ricky Gervais, Matthew Robinson, 99 mins, (12A)

Ricky Gervais's new American vehicle is ... I'm not sure what. Technically, I suppose, it's a satire, and it certainly has the makings of a scathing one – only it's just a little too benign. You'd really have to describe it as a high-concept comedy: that is, one dominated by a single idea pursued to its logical limits. But that's really another way of saying "an extended sketch", and The Invention of Lying essentially is that – a long, impressively clever sketch that gets hung up on being a sweet-and-sour romcom.

Still, the premise is neat. The setting is an alternative version of our world, in which it has never occurred to anyone to tell a lie. Consequently everyone is brutally honest all the time. "Oh, your baby is so ugly," smiles a woman at a new parent, and even a Coke ad flatly admits, "It's basically just brown sugar water." This is not only a world without lies, but one in which people and institutions alike are incapable of self-censorship: the façade of a care home carries the slogan "A sad place for hopeless old people".

Then along comes the very first liar: Gervais plays Mark Bellison, whose screenwriting job yields the film's most eccentric and inspired joke. If there's no lying, then there's no fiction either, so Hollywood specialises in historical lectures by dry professors in leather armchairs (Christopher Guest, in one of the film's several gratuitous cameos).

The basic conceit smacks a little of Ionesco, a lot of Monty Python: it's a verbal-philosophical gag, like the Pythons' Argument Office sketch. But then the film spins off in a more tendentious direction. Something goes ping in Mark's brain, and he commits the radically innovative act of telling an untruth: he instantly makes $500 profit. He finds that his new technique can even bag him sex: a woman rushes to a motel room with him because he claims that Earth's future depends on it (the flip side of universal honesty being universal credulity). It's the film's first fatal soft touch that Mark politely declines this sure thing: a blacker satire would have had him amorally seizing every opportunity.

In fact, lying soon ceases to be self-serving and becomes a creative, generous activity. Seemingly the only compassionate man alive, Mark starts telling fibs to make people feel better about life and death, and before long is holding a crowd enthralled with his preposterous gospel about a Man in the Sky who decides all our fates. Engaging in theological debate, people at last develop a sense of contradiction: why is it that the Man in the Sky rewards some, torments others? "So he's kind of a good guy, but kind of a prick too?" the crowd puzzles.

All this is not only witty, but also courageous in a mainstream American comedy – and it's Gervais's Englishness that lets him get away with it. Mark's status as an absolute outsider – the liar as the only lucid man – goes hand in hand with the presentation of him as a fish-out-of-water Brit, scorned by, but essentially superior to, all those gullible superficial Yanks. Because of the emphasis on Mark's/Gervais's character, the film comes across not only as a satire about truth and lies but equally as Gervais's own commentary on being British abroad; as such, it yields a sour, condescending vision of America as heartless and infantile. Mark Bellison may be pitched as someone whom the world merely tolerates, but Gervais's manner makes it clear that really the entire world is the fool that he doesn't suffer gladly.

The satire is mitigated throughout by a romantic strand in which Mark tries to woo the beautiful Anna (Jennifer Garner), who likes him but sees him as pudgy, snub-nosed, inferior genetic material. This joke might have been funnier if delivered by an actress who was comely but reasonably approachable-looking (Maggie Gyllenhaal, say). But when Jennifer Garner announces, "I'm out of your league," you tend to concede she has a point: she's Olympian in that scary Hollywood way, like a tree or a stylised effigy of a gazelle.

There's a very good scriptwriter involved in this film, and a mediocre director. One is Ricky Gervais; the other is Matthew Robinson, but I have no idea which is which – and each man could be a little of both. I don't know how they collaborated in this joint directing debut, but they've opted for a bland style that fatally dilutes the sharpness: flat, functional compositions and a twinkly retro score that tries to persuade you that you're getting vintage Frank Capra stuff.

This is an ingenious film that makes you chuckle, rather than gasp at its audacity – and you'd have to be a rabid fundamentalist to take anything like the offence that such a film should be capable of causing. And its life-affirming conclusion, tart though it is, plays right into the hands of the pious. Bet you anything there are preachers out there welcoming The Invention of Lying as a parable of how even an unreconstructed liar must finally embrace faith and family values – honest to God.