Wikus is a hapless bureaucrat at a military contracting firm, sent into District 9 to serve eviction notices to one million aliens ("prawns") who've been living in squalor for 20 years ever since their mothership broke over Johannesburg. His opinion of the aliens varies from annoying pets to dangerous pests, depending on how they're behaving themselves. In his enthusiasm for routing out illegal activities in the slums, he exposes himself to some alien gunk and begins an icky transformation, and suddenly everyone's after him - to cut him up, to kill him, even to eat him.
That's when heads start exploding. Alien weapons will do that. But the more interesting story is Wickus' journey from a bumbling, casually racist desk jockey to a desperate hunted man forced to depend on the aliens, one in particular, for help.
The movie is filmed in gritty documentary style, for the most part, using news footage and interviews to effectively convey a sense of realism. Oh, and did I mention that it's gritty? I love gritty.
Like Moon, District 9 is a low-budget movie that relies more on story and character than special effects. I live in hope that the upcoming James Cameron movie Avatar, which cost many times more, strikes the same balance.