Like a floating shopping mall, the alien mother ship has been hovering, broken and immobile, over the smog-shrouded skies of Johannesburg for 20 years.

The ominous spaceship just hangs there, blocking out the sun, while its passengers have long since been disgorged - human-sized, bug-like ETs living in what was originally a temporary holding zone but has grown into a militarized shantytown, a sprawling, barbed-wire-enclosed slum.

The premise for District 9 - filmmaker Neill Blomkamp's doc-style sci-fi yarn - is intriguing. The allegorical allusions to ghettos and concentration camps, the street signs reading "No nonhuman loitering," the callous degradation of an entire race: It isn't hard to see what this talented South African director is getting at.

But at a certain point in District 9 - with its frenetic jumble of mockumentary reports, talking-head interviews and tag-along TV news-like footage - the story of a midlevel bureaucrat contaminated by an alien virus devolves into just another video-game shoot 'em up. There's even a giant metallic exoskeleton robot thing - Transformers all over again.

The havoc (and the film) begins when it is decided to relocate the stranded aliens, derogatorily called "prawns" for their crustacean-like appearance, to an area outside Johannesburg. Crime is rampant, violence between humans and nonhumans is increasing, and the solution arrived at is essentially to sweep the whole thing under the rug. Or, more exactly, to move the encampment of 1.8 million aliens out of town, out of sight.

And so a private contractor, Multi-National United (MNU) - with its copters and armored vehicles, billy clubs and machine guns - is brought in to facilitate the evictions. Enter Wikus van der Merwe (Sharlto Copley), a midlevel MNU field agent. Wikus comes off as a bumbler, a bureaucrat, only moderately intelligent but well-connected - he's married to the daughter of MNU's boss.

At first, the evictions go fairly smoothly. The aliens are confused and wary, but they're signing the forms, grudgingly following orders. It's when Wikus discovers a shack full of beakers and chemistry-set gizmos - and opens a seemingly harmless canister (it looks like a coffee thermos) - that the trouble begins. Wikus starts to vomit black fluid. His fingernails flake off, and his hand metamorphoses into a tentacled limb just like the aliens have.

There's not much to be said about the acting in District 9 - the humans run, bark, scream, fight, and cry, and the nonhumans speak in subtitled radio-dial gobbledygook. What is absolutely impressive are the visual effects: the hordes of aliens, the mother ship, the seamless blending of the real with the fantastic. Peter Jackson, one of District 9's producers, turned over his Weta Workshop facility in New Zealand for the weapons, creatures, and makeup effects.

There is a lot of shield-your-eyes ickiness in District 9, a lot of violence and gore. What there is not a lot of, however, is humanity - even in the film's depiction of the inhumanity humans are capable of.EndText

Contact movie critic Steven Rea at 215-854-5629 or srea@phillynews.com.