A book is a book and a film is a film: they are different animals entirely. But Sarah Gavron's adaptation of Monica Ali's Booker-nominated first novel isn't so very different from its source material. It is less subtle and wide-ranging. Its characterisations are simpler. But its focus on the struggles of a young Bangladeshi woman saddled with an arranged marriage in the foreign city of London remains substantially the same.

Whatever the arguments against it - and we must remember that Stephen Frears's My Beautiful Laundrette was savaged in some quarters for suggesting that not all Pakistanis were heterosexuals - Gavron's debut is honest, sincere and sympathetic. In fact, it is the product of a first-time director who is clearly promising and probably more than that.

Tannishtha Chatterjee plays Nazneen, a 17-year-old girl who enters into an arranged marriage with London-born Chanu (Satish Kaushik). While she doesn't exactly rue the day, she does find her life transcribed by a pocket version of a paternalist society.

Chanu is no villain, but a man whose education is surprisingly wide-ranging even if his business activities tend to end up hopelessly inadequate. Still, he is not the right husband for Nazneen, and her attempts to start her own home sewing business leave him jealously guarding his own private world of supremacy. When she falls for Christopher Simpson's handsome young Kerim, who brings and collects the goods, Chanu becomes dimly aware that something is amiss but is uncertain quite what.

He stumbles on regardless, however, until 9/11 sends the drama into political as well as moral territory. Kerim is radicalised by local attacks on Muslims, Chanu is alienated and Nazneen is caught somewhere in the middle.

Gavron deals with the affectionate but routine relationship between Nazneen and her rotund husband with a sensitivity that doesn't preclude humour, and with the stumbling romance between her and Kerim with equal care. There are no heroes and villains, only people trying to live their lives as best they can and falling into the mire of difficult circumstances.

Both Chatterjee and Simpson are very good and Kaushik is excellent - a comical figure, perhaps, but not bereft of dignity. When he tells Kerim that Bangladesh was founded by Muslims fighting Muslims in reply to Kerim's protestations about Western perfidy, he suddenly seems like the true liberal hero of the film.

Does any of this slander the Bangladeshi community of Brick Lane? That is not for this review to judge, though this critic does not think so. It is a community that's as liable as the rest of us to get upset by criticism but one whose traditions deserve to be respected. The film, I believe, does this.

It could be said that the locations, transferred after the provocations away from Brick Lane itself, make the street look more lush than the physical reality. Gavron also makes the bad mistake of painting the scenes in Bangladesh with a glow that's simply not there, especially since Nazneen's mother is seen to drown herself during one of them.

Perhaps the director would answer that this is how Nazneen remembers her youth. But we shouldn't be encouraged to think that Bangladesh, one of the poorest countries in the world, basks in such vernal beauty. It's a tourist view of the Third World that's nowhere near reality.

On the whole, though, this first film, taken from a first novel, is quietly effective, acted with resource and directed with the kind of patient skill it needs. If you want more, you'll have to read the book. That will take longer than the 102 minutes we are allowed in the cinema.