REVIEWED - CLEAN: Olivier Assayas and Maggie Chueng have proved a potent combination in their two films together: the imaginative and engaging 1996 Irma Vep, made two years before they married each other, and Clean, shot in 2003, two years after their divorce.

Cheung deservedly received the best actress award at Cannes last year for her intense performance in Clean, and the film marks a welcome return to form for Assayas after his misguided previous picture, the exasperatingly flashy and vacuous Dreamlover.

Chueng plays Emily, who has been aptly described as a cross between Courtney Love and Yoko Ono. A former music television presenter, she lives with Lee (James Johnston), a faded 1980s rock star reduced to playing small venues. Both of them are hooked on drugs and, when Lee fatally overdoses in a grey Ontario town, she is found guilty of heroin possession and sentenced to six months in jail.

To Emily's dismay, Lee's parents (Nick Nolte and Martha Henry), who live far away in Vancouver, are placed in charge of her six-year-old son. When Emily is released, she desperately tries to sort out the mess of her life and to find the responsibility within herself to kick the habit, change her lifestyle and justify regaining custody of her son.

Given that Clean is a movie rooted in realism, Emily's road to redemption is a rocky one, paved with obstacles, chief among them her own weaknesses, fears and insecurities. Assayas wrote the script specifically with Cheung in mind, and she responds by immersing herself in the role, bringing Emily vividly to life in an admirably detailed and deeply touching portrayal of a woman teetering on the edge and struggling to claw on to the last vestiges of hope.

Nolte is effectively understated in the complex role of a man who blames her for the loss of his son while having to decide what is best for her child in this gritty, naturalistic and unsentimental drama.