The highly effective first act is presented as a documentary about the “alien situation." We learn that the aliens, who are derisively known as “Prawns” to the human population, suddenly appeared 20 years ago, hovering over Johannesburg in a broken down mothership that now hangs suspended like an intergalactic albatross, a high-tech reminder that humankind has transformed its first contact with otherworldly life forms into just another nasty round of Us vs. Them.

The moment when the government, after three months of waiting for some kind of signal, finally cuts into the hull of the ship to see who or what waits inside is supremely disturbing. The aliens, far from being mystical life forms, turn out to be terrified, starving masses cowering in their own filth.

Their ship is dead, and that’s it. They’re stuck here. So the South African government promptly shuffles the visitors into a heavily guarded shantytown known as District 9, where their guttural language and cockroach-like physical appearance only reinforces the idea that they’re dispensable irritants who couldn’t possibly coexist with human beings.

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But sometimes those attitudes can change. Blomkamp’s and co-screenwriter Terri Tatchell’s most daringly non-commercial move is to make "District 9"'s central protagonist a pencil-pushing nerd whose initial xenophobia is as repulsive as any displayed in the picture, and it takes him quite a while to get over it.

Wikus Van der Merwe (Sharlto Copley) is a perpetually grinning drone for a company called Multi-National United. M.N.U. has been recruited to move the Prawns to a relocation camp some 2,000 kilometers outside of Johannesburg, more or less so “decent” people will no longer have to look at them. Wikus’ father-in-law, a powerful member of the board of M.N.U., entrusts poor Wikus with the job of evicting 1.8-million Prawns from their shacks and preparing them for the trip.

This is easier said than done, since the Prawns rightfully distrust their human hosts, and, courtesy of their encampment, have collapsed into lives of seething anger and violence. District 9 makes Jamaica’s Trenchtown look like an afternoon in Palm Beach. The streets are littered with guts and garbage, and the Prawns spend the majority of their time either loudly arguing or fighting over rotting morsels of food. In as close to a joke as you’ll find in the entire picture, the Prawns have also grown addicted to the perceived delicacy of canned cat food, which is sold to them for a ransom by gangs of vicious Nigerian thugs.

Both the Nigerians and the government recognize the weapons the aliens had aboard their ship are powerful enough to enable any triggermen to take over the world. But the weapons aren’t operational if not melded with the aliens’ molecular structure; humans simply can’t fire them. So alien blasters are stockpiled by anyone who can get their hands on them, even though they’ve yet to figure out how to shoot the damned things.

The wild, nightmarish solution to this predicament eventually drives “District 9”’s plot…not that the solution makes a lick of sense if you actually stop to think about it.