In director Alejandro González Iñárritu’s second film, 21 Grams, just when gimmicks and an obvious plot foretell a sophomore jinx, the filmmaker does an about-face with his puzzle film.

The arresting, gritty film is fragmented, raising the bar on Pulp Fiction and Memento with its abrupt shifts back and forth through time. Watching 21 Grams is like gathering together the shards of a vase that has fallen to the floor. Everything’s fractured, but in the end, it’s all there.

And for that reason, Iñárritu’s framing device does not feel like a wanton gimmick. Good movies are made great movies by their endings and, all in the matter of several minutes, he winds up a film with as much revelatory power as was found in his debut, Amores Perros.

The less you know about the plot going in, the better. But like Perros, an accident is the connective thread to everything in the film, its aftershocks rocking the lives of two men and a woman.

Paul (Sean Penn) is a critically ill college professor, Jack (Benicio Del Toro) is an ex-convict with a temper to match his spiritual convictions in Christ and Cristina (Naomi Watts) is a wife and mother who regresses to her destructive life after the accident.

Even without knowing much, the narrative moves of 21 Grams soon grow easy to predict for even casual filmgoers. Because of that, the film teeters dangerously on the verge of playing all its cards and becoming uninteresting. But Iñárritu instead turns the lure into a thematic puzzle, parsing out the emotions and motivations of his characters, making for many knockout scenes.

There is a great confrontation between Paul and his emotionally estranged wife, Mary (Charlotte Gainsbourg), about the sham their marriage has become. Jack has furious discussions of religion with his zealous pastor, debating the difference between accidents and divine providence. And Cristina’s descent into drug abuse is without frills or flash and heartbreaking to see.

Each of these main characters is supremely well etched, creating their convictions and sticking to them throughout. It represents all three actors working at their apexes, a tough-to-earn compliment considering the powerhouse performances each has given in the past.

After years of playing tough guys (including this year’s Mystic River), Penn is a man totally uncomfortable with the idea of violence, burdened by the revenge mission he undertakes that’s fueled by love. His stunning, nuanced performance is, by turns, sad, sexy, charming and elegantly emotional — better than he was in Mystic River, although that work is the more likely to be recognized with awards.

Also continuing to turn on his tough-guy image is Del Toro, who again reveals a tenderness and vulnerability beneath his gruff, grimy exterior. His character’s spiritual crisis is gripping, as are the domestic peek-ins at his family life that feature fine supporting work from former Homicide detective Melissa Leo.

And Watts, unreasonably snubbed by most big-shot awards ceremonies for her work in Mulholland Dr., hopefully won’t get that same treatment for this role. Her character suffers perhaps the greatest mental collapse of the trio and her alternation of chilly introversion and love found at the perfect moment is brilliantly done.

21 Grams is a film filled with loss — loss of love, life, stability, fathers, mothers, daughters, faith, trust and self-esteem. And such seemingly inconsequential things as the color of shoelaces become signifiers for incredible heartbreak.

But in its brief, but crucial, balance of loss with gain, the finale is a thematic knockout of epic proportions. The question “What do we gain?” is asked at two crucial moments in the film, and Iñárritu and screenwriter Guillermo Arriaga’s answers are strangely and poetically positive and uplifting.

The film knows nothing about the muscular reflex of flinching, but it’s vividly knowledgeable of all other elements of the body — its physical and its mental, its life and its death. This is one of 2003’s best films.