The disparate threads of director Iñárritu's memorable Mexican drama Amores Perros were linked by a road accident.

In 21 Grams, his writer Guillermo Arriaga uses the same device to bring together the three protagonists of a sombre but equally compelling companion piece.

Iñárritu introduces his characters rather disconcertingly, flitting between events that occur at various points before and after the tragedy.

This approach takes a short while to get used to, but the film's rhythm is easy to pick up by tuning into the terrific performances.

Penn plays Paul, a heart patient being nursed by his English wife (Gainsbourg) while desperately awaiting a transplant.

Jack (Del Toro) is an ex-con whose dedication to God puts tremendous pressure on his family, whereas former wild child Cristina (Oscar nominee Watts) owes her personal salvation to her loving husband and daughters.

Fate intervenes and their destinies collide. Since the drama unfolds from the earliest scenes, further elaboration is unnecessary.

Taken singly, each performance would be a stand-out. Together, they are simply outstanding (though the irritating Gainsbourg is something of a duff note).

Working within an intriguing narrative structure, the actors ensure that the audience's attention never wavers.

Unlikely to raise many smiles, then, but a powerful and expertly told story that deserves all the plaudits it gets.

Elliott Noble

Science Note: 21 Grams is a great movie but don't get pulled in by the bad science. Bodies don't lose weight at the moment of death. For a full explanation in an excellent Guardian feature click here.