SPOILER ALERT - Do not read the following if you haven't seen the movie. It will ruin the experience.
Basically 21 Grams covers the lives of three different families and presents interesting conundrums and circumstances as the lives of these families collide with tragic consequences. The most interesting aspect of the film (and also the one aspect that makes the film so confusing to so many people) is the way Director Inarritu splices the events together. He basically scrambles the timeline so that the audience must become an active participant in putting all of the events in the correct order. We're not able to piece it together until the end of the film when all of the pieces have been presented to the us.
Following is a plot description in an order that makes the most sense:
The Three Families:
Family 1: The Rivers Family – Paul Rivers (Sean Penn) and his wife Mary (Charlotte GainsBourg). Paul is a mathematics professor with a heart condition. He will die soon if he doesn't receive a heart transplant. Paul continues to smoke despite his health issues.
Family 2: The Peck Family – Christina Peck (Naomi Watts) and her husband, Michael. The Pecks have two daughters, Laura and Katie.
Family 3: The Jordan Family – Jack Jordan (Benicio del Toro) and his wife Marianne. Jack is an ex-con who has turned "born-again' in a major way. Jack is convinced that God is in control of everything, even the smallest events in the World. Marianne is becoming tired of Jack's ways and does not approve of the way he is raising their two children.
The Events:
Christina Peck is notified by the Police that her husband and daughters have been involved in an accident.
Concurrently, the Jordans are throwing a party for Jack. Jack is returning from prison but when he arrives at the party it is observed by his wife that he is distraught over something. She sees that the front of his car is covered in blood. Jack tells Marianne that on the way home he hit two little girls and a man in the street. Jack wants to do the right thing but Marianne does not want him to turn himself in because she is afraid he will go back to prison and therefore not be around to raise their children. Jack turns himself in to the police.
Christina Peck arrives at the hospital where she discovers that her daughters and husband are dead. The doctors told her that had the authorities been notified in time, perhaps one of her daughters could have been saved.
Mary Rivers begins to accept the fact that her husband will eventually die. She wants to be inseminated by his semen, but Paul distraught by the fact that his doctor tells him that he probably won’t be around long enough to raise the kid.
Christina Peck is notified by the doctors at the hospital that there is a patient in dire need of a heart transplant. Of course this patient in need of the transplant is Paul River but Christina does not know who it is. Christina agrees to donate her dead husband's heart.
Jack Jordan is released from jail because Christina decides not to press charges against him.
After Paul's surgery he becomes curious as to who's heart he received in the transplant but of course, such information is held secret. Paul hires a private eye to ascertain the identity of his donor. Paul initiates contact with Christina several times but she is not interested as she is still distraught over her family's death. In one encounter, Christina takes too much drugs and therefore is unable to drive. Paul had followed her to the club and offers to drive her home. Paul, the next day, asks Christina for a date which she reluctantly accepts.
Paul is once again notified by his doctor that his new heart is beginning to fail. Paul goes through a long dilemma of whether of not to accept the new heart. He feels that it would only be prolonging death and he does not like the thought that it would only make him die better and deny someone else the new heart. He refuses the new transplant. Paul's marriage begins to disintegrate highlighted by an incident in which his wife announces to their friends that she will be receiving an artificial insemination. This angers Paul.
Paul has another encounter with Christina in which he sees a photograph of her husband (his heart donor) but can't bring himself to tell her that he is the recipient. Following a later encounter, in which Paul and Christina make love, Paul tells her his secret. This enrages Christina as she feels that only told her this to sympathize with her.
Paul's wife leaves him. He and Christina become a lot closer. She tells him that she wants to find out who killed her family. Paul once again meets with his private eye to ascertain the identity of the person who killed Christina's family. The PI gives Paul the information along with a loaded handgun.
Jack Jordan's home life has begun to break down therefore is living in a cheap motel. Paul and Christina check into an adjacent room to plan the killing. Paul goes to find Jack and takes him out into the desert where he threatens him with a gun and fires several shots miss Jack but seriously scare him. Paul tells Jack that it would be best if he were to disappear for good and never return.
Paul tells Christina that he killed Jack. That night as they lie asleep in their hotel room, Jack enters the room and begins a confrontation. The two struggle and Christina hits jack over the back with a lamp. A gunshot rings out. We learn that Paul, rather than shooting jack, shot himself in the heart. Jack drives the bleeding and dieing Paul along with Christina to the hospital. The doctors take Paul into the ER and jack tells police that he shot Paul.
Christina agrees to a blood transfusion for Paul but is later informed that her blood is of no use, due to high levels of intoxicants. She is also informed by the hospital staff that drugs are not good for the unborn baby in her belly.
We see that Jack is later released from the police due to lack of evidence.
The Conundrum: Paul says that we all lose 21 grams at the exact moment of death. He wonders who will be next.
P.S. the above comments reference the following movie review:
http://www.franksreelreviews.com/reviews/21grams.htm