Rosetta was a last-minute surprise at the Cannes film festival. With major names like Lynch and Almódovar in the running, why would the jury give the Palme d'Or to an unknown duo's vignette about working-class life in a drab Belgian town?

There may have been a political angle to the outcome - a vote against the mainstream and against auteur hubris. But finally, this low-budget feature won because it's a remarkably powerful film and an example of perhaps the toughest mode of all: hard-tack realism.

Brothers Jean-Pierre and Luc Dardenne have worked together for over 20 years in the town of Liège, but only made a name with their 1996 film The Promise. Like that, Rosetta deals with outsider life: its heroine is a teenage girl living on a trailer park and eking out a living for herself and her alcoholic mother. The film follows her from job to job, from disappointment to disappointment, the camera always tracking her at close range. She's not a great talker, so newcomer Emilie Dequenne, who won a best actress award at Cannes, has to convey nearly everything physically. Like a fine-tuned meter, the camera measures her fluctuations of mood from the tremors of suspicion and contempt on her face.

We may be tempted to see the determined Rosetta as some kind of everyday saint, but our sympathy is tested severely when she commits a simple but shocking act of betrayal. The Dardennes aren't just quasi-documentarists, but consummate storytellers, all the more so because they pretend not to be pulling any strings. Rosetta may be an unlikely Palme d'Or, but it's a salutary one - no arbitrary back-to-basics exercise, but a gripping example of committed film-making, and of the art of doing more with less.