The story begins when a ship full of sickly aliens appears over Johannesburg, South Africa. The aliens end up living in a crowded slum neighborhood called "District 9". Thirty years later, the aliens - derogatorily called "prawns" - are to be evicted and moved to a more isolated internment camp, with the move managed by a private military company (you too can can train to join MultiNational United and arrest non-humans!). Unrest and violence ensues.
The film's South African settings and depiction of the forcible re-settlement of the "prawns" into a segregated camp intentionally evokes on the historical enforced racial segregation of the apartheid era. But the racial issues are not just fictional; there has been significant criticism of the movie's depiction of Nigerian immigrants as gangsters and "savages".
Parktown prawn, a South African cricket pest. And like ants or bees the "prawns" are mostly drone workers. The disease that affected the alien ship apparently killed off the higher castes, presumably including their queen.
When the bureaucrat overseeing the eviction and resettlement process is infected with alien goo and starts slowly transforming into an alien himself, he is forced to find help among the District 9 inhabitants. That forces him to become more sympathetic to their plight, so as he becomes less human himself he can better appreciate the "humanity" of the stranded alien population.
But truly the science depicted in the movie doesn't make a lot of sense. At it's heart District 9 is a violent action flick, with an unusual setting (at least for a Hollywood flick). If that's what you are mood for, check it out on either YouTube or Crackle.
If you would rather watch a high quality version, District 9 is also available for instant download at Amazon.com (for a price, of course).
Image: Libanasidus vittatus or Parktown prawn. Image by Paul venter from Wikimedia Commons, shared under a Creative Commons Attribution Share Alike license.