THEY APPEARED in Johannesburg, and only there. Their filthy flying saucer, thousands of feet in diameter, still hovers, broken down above the city. Twenty years after their arrival, the aliens are penned up in District 9, a refugee camp. It was supposed to be a real home for them once, if you trust the decaying sculptures of humans and aliens striding hand in hand.

This apparently temporary camp is now an eyesore, a slum several miles wide. The aliens are idle but ravenous, living on theft, canned cat food, offal and old tires. The world calls them a racial slur, "prawns," because of the tiny vestigial arms coming out of their abdomens. But they're closer to 6-foot-tall cockroaches than to seafood.

In rapid, talking-heads introductions to Neill Blomkamp's District 9, we learn all this and more. We're watching a rough-cut documentary about the mysterious disappearance of Wikus (Sharlto Copley), a human employee of Multi-National United. Included in this mix of images is also some kind of private footage, heavily digitally watermarked, that a secretive company like MNU wouldn't want aired. This strange company combines the functions of a nongovernmental agency and a peacekeeping force—as if someone had privatized UNICEF and merged it with Blackwater.

The missing man is hard to like. A grinning sweater-vested underling, Wikus was promoted because he married the boss's daughter. (I call this obvious, because the act of nepotism is denied on camera.) And we see him during a raid on District 9, where he gets to play the tough cop for the documentary camera watching him. He shows the mercenary soldiers beside him how to kill a nest of prawn eggs. "Here's a souvenir of your first abortion," he says, giving an assistant a chunk of alien metal.

He and his uniformed team of thugs bust down doors to serve an eviction, so that the "prawns" can be relocated to a smaller and worse ghetto. During one raid, Wikus is sprayed with some alien fluid. After this incident, we can cut and paste in much of David Cronenberg's version of The Fly, as the smarmy Wikus begins to mutate, losing his fingernails and teeth. The massive corporation he works for makes him a wanted man, and he's forced to get the help of one of the prawns.

Copley's performance is small-scale to go with District 9's small-camera style, although Wikus turns into an acceptable action hero once he's infused with alien DNA. But Copley doesn't deliver the pathos during phoned-in messages to his wife. Apparently, director Blomkamp decided that it would be a violation of the documentary-camera rule to take a reverse angle so we could see shots of her during the poignant conversations.

Producer Peter Jackson's hand is visible in the flawless animation of the bug creatures and in the ultralow sonics of the special effects. The aliens have megaweapons and splatter humans into goop or throw them 20 yards in the air. But we never figure out why the aliens haven't used their arsenal to get the humans off their backs. The prawns aren't pacifists—they fight each other viciously. And they're not brutes; they have enough intelligence to communicate, and the Earthlings understand their clicking, wordless language.

Blomkamp seems to deliberately avoid the most interesting angles. Supposedly, some of the aliens are prostituting themselves; the rags they wear, loincloths and torn brassieres, indicate some kind of erogenous zones. And they piss from the same general direction as humans—fire-hose sprays of what looks like radiator antifreeze. But how does it all work? What kind of weird thrill seekers would copulate with creatures that could bite their heads off afterward?

These prawns have been through deep space, but no one seems to have asked the kind of questions any 6-year-old would ask about their culture—whether they had been enslaved or bred for work, for instance. All of the most interesting parts of this story—about the otherness of the aliens—are kept off camera. Ultimately, even technically adept films have to be answerable to the same laws of drama as something far more low-tech. How good—how profound—a movie would District 9 be if the aliens were played by guys in rubber masks?

The storytelling method is all jitter and no pulp. After Cloverfield successfully made an alien attack plausible in small-camera and cell-phone-video form, District 9's mock cinema vérité style is not so new. All the camerawork is shaky and careless, even when we're watching events that no documentary crew is there to watch. Having commentators speaking directly to the camera is the easiest, cheapest way to tell a story. It's also one of the flattest and most unsatisfying.

When all is said and done, Blomkamp has created a new slant on a promising story badly done in Alien Nation, whose invaders, like the prawns, arrived 20 years ago. Just as a rich idea of alien ghettos turned out to be a routine police story back in 1988, District 9 ends up as a familiar tale of alien infiltration, corporate skullduggery and very large guns.

Give Blomkamp credit, he did have one inspiration: figuring out how to make a science-fiction film against the appalling background of Soweto. For better or worse, he delivers a sociological Other, in the way the early Australians saw the aborigines, or the early American saw the Indians: as nonhumans, as vermin, as bugs.

DISTRICT 9 (R; 112 min.), directed by Neill Blomkamp, written by Blomkamp and Terri Tatchell, photographed by Trent Opaloch and starring Sharlto Copley, opens Aug. 14.

Send a letter to the editor about this story.