(CNN) -- Why do UFOs always land in the USA, wondered someone in "Monsters vs. Aliens"? Good question.
Sharlto Copley plays a bureaucrat forced into action in "District 9," a film about extraterrestrials.
But it's not always the case, as Neill Blomkamp's adventurous sci-fi movie, "District 9," goes to show.
Presented in the now-familiar fake-doc style -- at least until Blomkamp gets bored with its limitations when the action heats up -- "District 9" is an edgy little B movie with an injection of impressive big-money special effects courtesy of "Lord of the Rings" director-producer Peter Jackson.
The story's origins are with the extraterrestrials' arrival in the early '80s, when they park their spaceship a mile or so above Johannesburg, South Africa. The ship is vast -- almost the size of the city -- but when after three months there has been no contact from the visitors, no sign of life in any form, the authorities decide to break in.
What they find is a wretched cargo of pale, starving creatures: tall bipeds with scaly skin and stubby insect-like tentacles around the mouth. The humans call them "prawns," and after establishing that the worst threat they pose is a cosmic PR disaster, they bring them down to earth and stick them in a refugee camp.
That would be District 9, a shantytown of makeshift wooden shacks, no electricity or running water, and scarcely enough space for the million or so newcomers.
After a couple of decades of uneasy coexistence, even this set-up seems too cozy for the residents of Jo'burg. With civil strife and black-market gangsters getting out hand, it is decided the prawns should be relocated to a new camp, somewhere out of harm's way.
International law being what it is, the South Africans are required to collect the signatures to indicate the prawns' acquiescence. And that's where mild-mannered bureaucrat Wikus Van De Merwe (Sharlto Copley) comes in -- with a delegation of trigger-happy private security guards.
"District 9" is a truly global affair. The South African-born Blomkamp learned his trade in Canada, and found a mentor in New Zealand. But it's the location that allows this allegory to resonate.
Here in North America we may darkly imagine that interlopers not only mean us harm but that they have the wherewithal to back it up. In South Africa, though, the story takes on more complex, queasy echoes. The British effectively invented concentration camps here at the turn of the 19th century, and the Boers, the victims then, would mandate that native Africans were nothing but second-class citizens.
Naturally the prawns come lower still down the pecking order. Blomkamp indulges in grisly, blackly comic satire worthy of Paul "Starship Troopers" Verhoeven as our man Wikus (Sharito Copley) casually demonstrates the art of aborting alien offspring with the aid of a flamethrower, all for the edification of the viewers back home.
Complacent and overconfident in front of the cameras, Wikus gets his comeuppance when he accidentally ingests an unidentified liquid substance, which incurs an alarming physical reaction. At this point, the movie changes tone, sometimes for better, sometimes not.
"District 9" also begs plenty of questions; not much of it makes sense if you stop to think about it. And Blomkamp seriously overplays his hand in the long, thoroughly conventional climactic shoot-'em-up. What should have been a taut 90-minute nightmare flick -- a classic midnight movie -- has somehow pumped up into a 112-minute approximation of a Hollywood action blockbuster.
Still, "District 9" has plenty of material to chew on. Expanded from a six-minute short, it's a brashly confident debut, full of sharp, inventive detail (the prawns are crazy for cat food) and rooted in a couple of Big Ideas. Well worth the time, if not all of it.
"District 9" runs 112 minutes and is rated R. For Entertainment Weekly's review, click here.