Like Arriaga's and González Iñárritu's previous film, Amores Perros (2000), 21 Grams interweaves several plot lines, around the consequences of a tragic automobile accident. Penn plays a critically ill academic mathematician, Watts plays a grief-stricken mother, and del Toro plays a born-again Christian ex-convict whose faith is sorely tested in the aftermath of the accident.
21 Grams is presented in a non-linear arrangement where the lives of the characters are depicted before and after the accident. The three main characters each have 'past', 'present', and 'future' story threads, which are shown as non-linear fragments that punctuate elements of the overall story, all imminently coming toward each other and coalescing as the story progresses.
- Abusive Parents: In an establishing moment, Jack not only lets his son hit his daughter but forces to her to take another hit, quoting scripture. Then hitting his son for his act to begin with.
- The Atoner: Deconstructed with Jack. He genuinely wants to do good and atone for his actions but comes across as condescending, hypocritical and selfish, not really thinking how his actions will affect his family.
- Fake American: Naomi Watts as Christina.
- Heel Faith Turn - Jack Jordan (Benicio del Tor), but whether he's really better than before becomes a critical question of the film.
- He Really Can Act: Naomi Watts is the most surprising of the trio (Having mostly done small projects beforehand) but Benicio Del Toro arguably gives the best performance (He had already won an Oscar so wheter it counts is up for debate).
- Holier Than Thou: Jack.
- Billy Needs an Organ - Paul Rivers (Sean Pean)
- Off the Wagon - Christina Peck (Naomi Watts)
- Stalker with a Crush - Paul follows Christina around for a while without revealing how he knows her.
- Tear Jerker: Many moments but Christina's plea for Paul to help murder Jack stands out.
- Anachronic Order