THE MOST PRETENTIOUS part of 21 Grams is its ending – which is quoted whole in the trailer. It's a monologue spoken in voice-over by Sean Penn as we catch a few last glimpses of all the major characters in their variably fragile circumstances. He notes the human body upon death loses 21 grams – the title explained at last – "the weight of a stack of nickels, of a hummingbird, of a chocolate bar ..." He then ponders several eternal questions: Why are we here? What is a soul? Of course there are no answers; we're meant to tearfully shrug – yes, isn't life just that mysterious and precious.

It's a self-consciously literary moment, one that's out of sync with the mercifully unmetaphorical preceding two hours and adds little beyond a forced "grace" note. Still, it's also the sort of gratuitous gesture that works for many (like Academy voters), underlining a project's importance. But seeing them fly a "Warning: Profundity at Work" flag this way is like finding passages highlighted (with margin note: "So true!!!") in a used book whose depths you'd have preferred to recognize for yourself.

21 Grams is a good movie hobbled most by its certainty of greatness; its entire construction, nonstop emotional urgency, and near complete lack of humor signal as much throughout. It's better than most "prestige" efforts – certainly the concurrent Penn vehicle Mystic River, which similarly orchestrates several personal tragedies into contrived sentimental-existential narrative symphonies – due to the makers having one foot in art-house cred and another in starry Hollywood uplift.

Amores perros director Alejandro González Iñárritu and scenarist Guillermo Arriaga should be congratulated for making a film that was first conceived for Mexico City seem not at all awkward in the English-language U.S. milieu. There's a grittiness of tenor and texture that's brave for a commercial film – it has the most unhealthy visual palette since Traffic, which suggests hollow-cheeked costar Benicio Del Toro has overall aesthetic jaundice written into his contract. 21 Grams is so frequently so good on a scene-by-scene basis that one wishes only it hadn't gotten some very big ideas.

The clever scrambled-chronology format (there's no "present tense" here, though certain late events are progressively unveiled) keeps us on our toes in terms of the developing connections between lead figures. Ex-con Jack (Del Toro) is struggling to stay off booze and be a good supporter to his wife and two small children. Cristina (Naomi Watts) is a suburban housewife whose world collapses when her own spouse and young offspring are killed by a hit-and-run driver – Jack, we soon learn. College prof Paul (Penn), his marriage to Mary (Charlotte Gainsbourg) failing, becomes obsessed with Cristina – widow to his late heart-transplant "donor."

These individual tragedies and their survivors' crooked paths through the various steps of grief (numbness, guilt, rage, etc.) form 21 Grams' asymetrically weaved tapestry. Unlike the more elevated-pulp-style Mystic River, it isn't entirely hinged on vengeance; the psychological deterioration of the protagonists gets more than equal weight. Watts is poorly back-storied – and as she plays it, more convincing as a drugged-out party girl renewing old bad habits than she is as a loving wife and mother in mourning. Still, her Sturm und Drang scenes have a downbeat potency. Del Toro benefits from a more thoroughly imagined character arc; Jack's self-loathing, born-again zeal is as immediate as the tough love with which his wife (Melissa Leo, who's terrific), a very reformed-biker-chick, treats him.

Penn feels wrong as a university intellectual – he's never been able to communicate bookishness. But he's marvelous as a man impaired to near stasis by an oxygen mask and post-op frailty. The physical restraint imposed by the role brings out his most nuanced acting; it's a great contrast to Mystic River, in which he seems a Method cliché of bruiser eloquence. (His "big scene" in that film, bellowing wounded-animal protest behind the police line around his daughter's found corpse, pretty well defines overly set-up, Oscar-bait overacting.)

21 Grams is bleak, inventive, and heartfelt to degrees that feel right until they don't. I wish filmmakers didn't feel the need to construct entire features as if artistry meant wedging all of life into two hours. This one has the same pompous circus-of-fate, micro-"cosmic," whirling-contrivance feel that many others of similar bravado do (Magnolia, The Hours, Talk to Her, The Royal Tenenbaums, anything by Frank Darabont). Previous generations used to call it "Capra-corn." It's still overarching, and it's still (variably) good art pushing the envelope of auto-award-handing bogusness.