Brainy sci-fi thrillers are always a treat, especially in a summer like this one, where the popcorn tentpole movies have for the most part been fairly uninspiring duds. So let's be thankful for Neill Blomkamp's remarkably assured feature debut, District 9, which although hardly flawless, rips along at a devilish pace unheard of, to my knowledge, in metaphorical commentaries on Apartheid.

The situation, in a nutshell: in 1982, a very large alien vessel set to hovering in the airspace above Johannesburg. When it was cut open by local soldiers, it was found to be full of thousands of sentient insectoid beings, all of them desperately undernourished and near death. Skip ahead to 2010: those aliens - mockingly called "Prawns" - have been set up in a wretched shantytown called District 9, and despite hints of early attempts to coexist harmoniously, have been reduced to the most impoverished, slavish kind of second-class existence: unable to leave their compound, they eke out a miserable life scavenging and eating canned cat food as though it were the finest substance known to man. District 9 is administered by a private weapons development firm with the terrifyingly Orwellian name of Multinational United, or MNU, and as the film's story begins, they have apparently decided that being kept in a segregated ghetto isn't good enough; to make room for further expansion by Nice People, MNU has built a ghostly village of flimsy tents more than 200 kilometers outside of town, and is preparing for the imminent removal of all the aliens to this godforsaken remote concentration camp.

You don't need a PhD. in contemporary political history to see the parallels Blomkamp is trying to draw between his scenario and Apartheid; and while it must be acknowledged that he doesn't end up doing very much that's very interesting with this parallel, at least it speaks to a good-faith effort to make an effects-driven blockbuster tethered to legitimate human experience, rather than a blazingly amoral explosionpalooza like - ah, but I have picked on Michael Bay too much this summer - so let's instead say, like Roland Emmerich. It helps immensely that District 9 was shot in the honest-to-God shanties of Johannesburg, giving it that much more verisimilitude and serious "oomph", if you will.

It also helps that the characters are well-written and well-performed, especially the protagonist, Wikus van de Merwe (Sharlto Copley), the son-in-law of the head of MNU (Louis Minnaar). He was put in charge of the relocation project despite a total lack of qualifications, in a crisp example of nepotism at its worst; he quickly learns the dark side of MNU and District 9 when he sprays himself with an alien substance that starts to convert his DNA to that of the aliens. Wikus is an exceptional kind of hero; because neither the script nor Copley ever stops to ask us to like him, or even find him more than borderline tolerable. He's a callow idiot when we meet him, and his experiences leave him fairly angry and scared and maybe a bit crazy; but by God, he's never once a very pleasant, lovable man, although plainly his family loves him. That's their problem.

For the most part, District 9 is structured as a documentary, collecting footage from the previous 28 years and plenty of interviews with enough people that I gave up trying to keep them all straight within minutes; all we can tell at the start is that something happened that apparently caused great controversy, and that Wikus has been given up for dead. It would be a lie to claim that the documentary format is hewed to perfectly in all cases, but Blomkamp makes a real fine attempt to keep that reality alive for a good thirty minutes at the beginning, and intermittently thereafter. This is the aspect of the film that best demonstrates his imagination and skill: the depth of fine details that go to giving each "type" of footage a discrete personality speaks very highly of his commitment to creating this world - and it is a remarkably well-defined world, for all the numerous open questions that remain at the end of the movie (chiefly, what the hell brought the aliens to Earth? - but since the human characters never learn this detail, I don't mind that we don't either), perhaps the most fully-realized science fiction reality since Children of Men.

The documentary conceit also works wonders to help along the film's single best illusion: the aliens themselves. They are wholly an invention of the computer, but only in a small handful of instances do they ever come across as CGI (mostly due to shoddy compositing). The overall appearance of the film is sufficiently low-fi, and the effects are correspondingly low-resolution, and the marriage of the real footage with the animation is unnaturally convincing. Besides that, the design of the aliens is pretty damn unique and persuasive, although like most movie aliens, they are inexplicable bipedal and human-shaped.

As a thriller, which is how it clearly desires to be taken, regardless of the feints towards Apartheid satire, District 9 is never unsuccessful, which is, pointedly, not the same as always successful. There are slow patches that don't seem to do much besides stretch a 90 minute film to within ten minutes of two hours. But the parts that work are just splendid; particularly the early relocation scene, in which Wikus and his small army tour District 9 attempting to inform the residents that they will be moving in 24 hours. The big finale sequence suffers a touch from being too "big" for the rest of the movie, but it's fun enough in an adolescent male sort of way that I'd just as soon not make a big deal of it.

But it does have its share of problems, and they're big enough to make me wonder how the film came by its glowing buzz. For a start, it's racist. Yes, it's anti-Apartheid, but that isn't exactly a cutting-edge progressive stance any more. And the black characters in the film are mostly of a deeply unfortunate caliber: black marketeers and cannibals - actually, strike that, I guess a human who eats an alien isn't a cannibal, technically. But they are savages on the model of a 1930s adventure serial, and colossally out-of-place in a movie made in and about South Africa in the 21st Century.

The other really big problem is the story structure; it doesn't really go anywhere. A lot of that is because Blomkamp isn't trying to tie everything up with a pat resolution, and that's something I completely support; also, the documentary structure lends itself to "this is what happened, but we don't quite understand" storytelling. But when you get right down to it, the drama doesn't really have any stakes - not about Wikus's fate, and not about the aliens' right to live a civilized life. By implication, these issues are the ultimate point of the story, but only in a vague, sort of way, like how shitty B-movies from the 1950s would end with "The End...?" in lieu of resolution. As it stands, District 9 is a massive tale with dozens of dead bodies, all about getting one person in position to do one thing, and the effects of that aren't just shunted off the table, they haven't even begun to happen by the time the film ends. In the end, it signifies nothing.

Still, its sound and fury are undeniably impressive, particularly given its relatively modest $30 million budget (an amount that producer Peter Jackson allegedly gave to Blomkamp with the admonition "make a movie with this", in an effort to get himself a real live protégé). It is not the masterpiece its loudest fans want it to be, but it's a fun movie, and better still it is a fun movie that does not shy away from intelligence. In other words: it's one of the best summer films of 2009, and not only because that particular bar was set so low.