At first blush it would seem that there's no more hidebound movie genre than the Western, which has been with us since the dawn of narrative film and has accumulated as many cliches as it has tumbleweeds.

But in fact, as the quintessential American form, the Western has shown a remarkable elasticity. The Taiwanese-born, American-educated director Ang Lee comes to his new film, the post-Western Brokeback Mountain, having already made a pre-Western with the 1999 Ride With the Debi.

Brokeback Mountain is a lush, haunting, bittersweet film about love and loss and the impossibility of being true to one's desires. You might go into the movie expecting to snicker at its stars or at a Hollywood intent on politicising even the most sacrosanct of film genres. But you're likelier to come away stirred, haunted and deeply moved.

The film opens in 1963, when Jack (Jake Gyllenhaal) and Ennis (Heath Ledger) are hired to spend the summer looking after a flock of sheep on Brokeback Mountain. For reasons of secrecy-they're illegally keeping the animals on federal land-they're ordered to spend the nights apart: one in the high country with the sheep, the other at a designated campsite. The two are rough-hewn, almost stereotypically so.

But when they separate at summer's end, there's an unspoken poignancy thick in the air. Ennis goes home and marries, Jack hits the rodeo circuit and finds a wife as well -a rich one in Texas. But they both think back to Brokeback Mountain, especially Jack.

After a few years, he starts making regular visits to Wyoming so that he and Ennis can return to the mountain, telling their families they're fishing. Over time-more than a decade-their lives change but they keep revisiting the scene of their youthful love, grasping for something that brought them happiness and which they could never fully own, not in this life, not in this world. It's a simple story but it's elongated to more than two hours by Lee with an emphasis on gorgeous photography and a laconic musical score by Gustavo Santaolalla.

And that's as it should be, really, as the film is about the most human of all impulses and how it once struck an unlikely pair in an unlikely place at an unlikely time and how it determined the rest of their lives by its absence or, more precisely, its fleeting recurrence. Beautiful, poetic, mournful, at once rich and spare, Brokeback Mountain takes a daring conceit and creates of it an overwhelming work of art that should speak to anyone capable of love.