On the surface, "21 Grams" refers to the alleged amount of body weight someone loses after they die. As in real life, such an explanation is not nearly as simplistic. An attention-demanding, heartwrenching drama about three unconnected people brought together in unpredictable ways following a fatal car accident, auspicious filmmaker Alejandro Gonzalez Inarritu (2001's "Amores Perros") penetrates into the darker corners of the human condition. Following the title theory, he further questions exactly what those 21 grams hold. Is it the weight of the human soul? Is it the weight of a person's sins? The guilt they have harbored over the course of their lives? And, furthermore, is redemption a possibility in life when everything you have known suddenly crumbles down around you?

Until its revealing final moments, these questions (and their answers) hang over the film with a palpable dread of increasing hopelessness. Told completely out of chronological order and designed with a level of expert complexity as revelatory as any motion picture this year, director Inarritu and screenwriter Guillermo Arriaga thrusts his unsuspecting viewers into the middle of the three lead characters' lives, interweaving the past and present to form a complete and satisfying narrative. The unconventional style may be disconcerting at first, as scenes from wildly varied moments in their lives are shown at apparent random before any sort of story has unveiled itself. At the half-hour point, however, as the stakes are accelerated and the puzzle pieces begin to reveal themselves, the film finds its groove. By the end, a conclusive storytelling whole has been achieved out of a wholeheartedly one-of-a-kind technical style. The result is as emotionally devastating as it is cinematically rewarding.

The overriding power and incalculable plot developments that "21 Grams" ultimately holds makes it difficult to discuss in detail; giving too much away would be criminal to prospective audience members. What can be said is that Paul Rivers (Sean Penn) is a dying man with a serious heart ailment whose wife, Mary (Charlotte Gainsbourg), wants nothing more than for him to artificially inseminate her. Cristina Peck (Naomi Watts) is a once-stable mother and wife who has become unsocial and deeply depressed, masking her severe pain in heavy-duty drugs. And Jack Jordan (Benicio Del Toro) is a family man who finds his devout belief in God and faith suddenly being put to the test. The catalyst for these life-altering experiences is a car accident that, in dissimilar ways and under different circumstances, deeply affects Paul, Cristina, and Jack.

The leading trio of performances rank as some of the standouts of the year. Leading the pack is Naomi Watts (2001's "Mulholland Drive"), courageous and unflinching in her portrayal of Cristina, whose appearance and psyche metamorphoses in such stark ways that it has to be seen to be believed. The emotions Watts evokes on her journey are so raw, and she handles them in such honest ways, that some of her more dramatic moments are difficult to even watch.

In a vast improvement over his overrated turn in "Mystic River," Sean Penn is sheer perfection as Paul, a man wrestling with his own dwindling mortality who finds life in the most unexpected of places. Penn is unusually understated in his portrayal, and his low-key approach to a person who tries with all his might to keep his emotions at bay is astonishing in its effectiveness.

As Jack, an ex-con and Catholic who has just begun to put his life back together when an abrupt event leaves him spiraling down into a sea of unbearable guilt, Benicio Del Toro (2003's "The Hunted") delivers a focused, imploding, frighteningly tragic performances as remarkable as his Oscar-winning work in 2000's "Traffic." The road Jack ultimately decides to take, which he at first believes to be the most virtuous under the circumstances, only makes his guilt worse, alienating him from his wife and children in the process. The unblemished supporting cast includes Charlotte Gainsbourg, as Paul's wife, Mary; Melissa Leo, as Jack's frustrated wife, Marianne; and Clea DuVall (2003's "Identity"), as Cristina's concerned younger sister, Claudia.

Above all, "21 Grams" is a fascinating, existential study of the way a single event, for better and for worse, can irrevocably affect people's lives. While one person loses seemingly everything because of it, another is given a second chance at life. After all the cards of fate have been dealt, can one possibly pull themselves out of the sort of despair for which there doesn't seem to be a possible remedy? And is the good fortune of another truly good, or simply a temporary stay of execution? While the purpose of the film's unchronological design is sketchy at best, there is no doubt that it wouldn't have held the same piercing impression without it. "21 Grams" is a challenging motion picture, and one that may require a certain amount of patience from those viewers used to having their stories and characters handed to them on neat silver platters from the very first scene to the last, but the reward for sticking with it is an imaginative, heartbreaking film—one of 2003's best—that you won't easily forget.