District 9, Neill Blomkamp, 112 mins, (15)

South Africa-set science-fiction oddity District 9 is as close as mainstream cinema gets these days to a cheap and cheerful B-movie.

That is, it's clearly not cheap – its producer is Tolkien-adapting mogul Peter Jackson – and it's cheerful only in the grimmest way. But it does have a punchy idea or two, a dash of sour wit and some flamboyant action style: if there were still drive-ins across America, it would be playing in every one. As it is, it has done very nicely at the US box office, netting $91.4m (£56.3m).

So Neill Blomkamp's polished production isn't quite in the rough'n'ready bargain-basement school of Roger Corman – although it does have something of the inventive grit of Corman alumnus James Cameron in his early days. District 9's premise is quite an original one, too, if you haven't seen the 1988 film Alien Nation, that is: space beings arrive on Earth and settle into the role of unloved immigrants.

Blomkamp's take on the theme yields, at least initially, an acerbic parable about apartheid. The film starts in familiar mock-documentary vein, as a report on the problems surrounding extra-terrestrial presence in, of all places, Johannesburg. Some 20 years earlier, a vast spacecraft appeared in the skies above the city. It turned out to contain a race of crustacean-like aliens, leggy things with twitching mandibles and burbling voices.

Humans at first succoured the stranded visitors, housing them in a hastily erected refugee camp. But the humanitarian impulse quickly soured, and now the newcomers are a loathed underclass, known by the contemptuous nickname "prawns". They are scorned by rich and dirt-poor, whites and blacks alike. However, Blomkamp – even-handedly, you might say – goes out of his way not to make the aliens obviously sympathetic: they derail trains for fun, casually chomp tyres as if they were marshmallows and, ominously, are stockpiling fearsome weapons that only they can operate.

Conversely, however, there's barely a likeable human in sight. The film's ostensible hero is a grinning dolt named Wikus van der Merwe (Sharlto Copley); he works for MNU, a company that plans to relocate the aliens to a new camp, where they need trouble Jo'burg's population no more. In fact, MNU is a ruthless weapons corporation interested only in acquiring the alien armoury, but the clueless Wikus doesn't know that, and eagerly signs up for a high-profile ethnic cleansing operation. As bigoted as any other human, he relishes his moment in front of the documentary cameras, enthusiastically explaining how prawn eggs burst like popcorn when torched. So we can't muster too much sympathy when he gets a splash of unidentified goo on him and unspeakable things start to happen, the sort of things that David Cronenberg would approve of.

Blomkamp and co-writer Terri Tatchell do a brisk job in the first half hour, using the mock-doc format to present the aliens' presence on Earth as perfectly everyday, while the humans expose their bigotry with cheerful dunderheadedness, and worse. The aliens' shanty-town deprivation is vividly evoked: the ungainly creatures stagger around, tormented by a junkie craving for cat food, for which they're willing to pay exorbitant prices to ruthless Nigerian crime gangs.

But when Blomkamp abandons the documentary framework to follow Vikus's misadventures, the film shifts into pacy but routine sci-fi action mode. The familiar dystopia-story premise – conformist unwittingly finds himself on the rebel side – is lost amid a barrage of shoot-outs, gelatinous splatter, and gratuitous displays of fancy hardware.

Representing the aliens is one "Christopher Johnson", played by Jason Cope – although whether this means that Cope just does the voice, or provides the twitchy, juddery movements for this chimera of CGI and prosthetics, I don't know. Johnson comes across as the most human character on show – although in saying that, I suppose I'm showing typical human prejudice. But the dice are loaded in favour of this prawn resistance fighter, the story's only fully characterised alien. What's more, Johnson has an alien child, a winsome little thing and CJ senior's easy ticket to our sympathy: how can we not root for a loving crustacean paterfamilias (a prawnfather? a crawdaddy?).

Sharlto Copley tries a little hard as the rebarbative Wikus: he certainly grabs our attention at the start, although his characterisation is essentially a ratty, rabid, flustered version of David Brent. But as the action hots up, you quickly tire of Copley's main trick of spitting out a panicky "Fook! Fook!" in a dense Afrikaans accent.

Director Blomkamp is good at mustering speedy action and posting grisly sight gags in the background, carnage always hovering comically at the edge of your vision. But I'm not sure that District 9 entirely gets the earthling racism out of its own system: its black actors generally get minor roles, unless they're playing the feral Nigerians, whose leader is after alien flesh to devour for its power-giving properties.

The genre slapdashery wins out over satirical finesse, but still District 9 is a provocative curio: the only film ever to put the "E.T." into "Soweto".