Twenty-one grams is the weight supposedly lost by the body at the moment of death: the weight of the soul. And "21 Grams" is about a group of people who are the living dead; in the course of the film they lose their souls, and try as they may they cannot recover them. They are brought together by a horrifying accident, which in director Alejandro González Iñárritu's puzzle-box structure does not occur until some way into the film. Though he builds on an accident and its effects, as he did in his first film, "Amores Perros," we meet and know each person as they both were and are, before it and afterwards, even before he tells us exactly what happened. In lesser hands that structure would make a hash of his story and, I think, destroy the film. As it is, it barely survives his manipulation. We have the uneasy feeling that he thought his film would not have enough power were he to show us what happens in a conventional manner; and so decided to shatter his artist's mirror and give us the shards and detritus as he puts them haphazardly back together.

Sean Penn is mathematics professor Paul Rivers, married to Mary (Charlotte Gainsbourg). He is dying of heart failure and waiting for a transplant. Benicio Del Toro is the alcoholic ex-con Jack Jordan, trying desperately to save himself and his family by latching onto an evangelical Christianity that consumes him and them. And Naomi Watts is Cristina Peck, whose family has been, and will be, lost. The strands of each and all are woven back and forth in time by González Iñárritu and his collaborator, the writer Guillermo Arriaga, who also wrote "Amores Perros," but even at more than two hours the film seems underwritten.

This is particularly true of the failing marriage of Paul and Mary (Penn and Gainsbourg); there is little to go on, and the revelation behind the breakup, when it comes, is too trivial in this day and age to be worth our interest. Moreover, if men are defined by their work, he would be more believable as a teacher of sixth-grade arithmetic than college mathematics. Only one bland and clichéd scene gives us an inkling that he's acquainted with his field, and that only tangentially. And then: For some reason this magnificent actor has made the dreadful choice of looking like the aging hood he played in "Mystic River." His greasy pompadour belongs on another character entirely.

Del Toro, on the other hand, is extraordinary. He has the look and the aura of a wounded saint, and at the same time of a divinely cursed human. He uses his big body as though he were a linebacker for God, albeit a god who has cursed him, and through him his family. Though he is the cause of great pain he is also a victim, and his performance is brilliant.

And then there is Naomi Watts. With every film she reinvents herself, creating a new human being who belongs only in that role, who is no more and no less than that person. Here in "21 Grams" she is given an almost unplayable part: she is a narcotics addict, good if guilty wife, surviving lover and murderous avenger. She manages to be all of those at once, in a miracle of a performance. Watts is very pretty but she does not have the striking beauty that so many other actresses can rely on; in each film she must find a new way into her character, and then turn herself inside out so we too may see into her. It is a great gift.