A bunch of wealthy neighbors live in a closed compound in Mexico City, where they have created their own little world in which the only contact they have with the rest of society is through their servants. They have their own security force, their own chapel, even their own school. The rich kids are sheltered from the poverty that surrounds them in the slums that proliferate right outside their walls. They are sheltered not only from the poor, but from reality. They live in a bubble in their own country. In Mexico City there are many such residential developments and neighborhoods where people just close off their streets and hire private guards, so nothing sci-fi there.
(By the way, I'm dying to know how the privileged audiences reacted to the film.)
Except for the detail of the school, which is a slight exaggeration, all this is already happening in Mexico City (as it is in Sao Paulo or Caracas, or you name it).
Three young punks from the neighboring slums come into the gated community to steal and things go awry. People are killed. Because of the wealthy Mexicans' not unfounded fear of the police, and because of their rampant paranoia about crime, these rich people decide to take matters into their own hands rather than calling the law, which is so blemished by corruption that everybody thinks they are above it, particularly the rich, who buy it on occasion.
This is the part of the movie that stretches credibility for me. I think that what the rich would actually do would be to call their friend of a friend who has a friend in high places and pay the police off to do the dirty work. They'd be too lazy and squeamish to do it themselves. However, I can understand that there are fictional, dramatic requirements that need this kind of poetic license, and the film, to its credit, makes it work.
The film takes the crux of life in Mexico and extrapolates it to a microcosm that reflects it pretty accurately. There are insufferably arrogant rich people (who are not an exaggeration) and there are people with some sort of conscience but with too many private interests or just too much comfort to really bother.
And then there is the police. When they come to investigate reports of gunshots, the rich immediately try to buy them off (an instant reflex, perhaps). The commanding officer doesn't take the bribe, not because he's beyond reproach (no one is) but because he feels insulted, and with reason. As he decides to investigate, at first just to spite them, then with some principle, he of course runs into venal corruption from above, not only rendering all his efforts useless, but creating even more appalling injustice, which victimizes the truly innocent.
The film is clear eyed and does not leave people off the hook; there are no unimpeachable heroes (no one would believe that, since Mexico is a country of cynics), just people struggling with their notions of right and wrong and how they can be applied to such unfairness. Yet everybody's choices are marred by corruption so deep, nasty and pervasive that they are up to their necks in it. Some of the images are strong and effective metaphors: a golf course surrounded by hills rife with slums, the rich disposing of the bodies of the poor in garbage bags, a chase scene inside a sewer (echoes of The Third Man) that ends still inside the fortress of the compound. There is no way out.
In the end, justice is not served, (this would truly be a fantasy); there is only the awakening of conscience in each individual, which should be taken as a triumph. The final image of the film is of a rich teenager finally escaping his enclave, right into the midst of what his parents were trying to shelter him from all along. He eats some street tacos in the dark in the middle of the slums. When I saw the taco stand, with its two hardworking taqueros bathed in golden light, I knew he was safe. I saw redemption.
This is what you are missing, rich people, the movie seems to say, and this is who you are insulting with your contempt and your racism: the country that feeds you, literally and symbolically.