The idea is that a giant spaceship is hovering over Johannesburg. Eventually it's boarded and the occupants (called prawns because of their resemblance to, well, prawns) and installed in District 9, a slum on the outskirts of the city. The movie begins in a pseudo-documentary style on the day when the prawns are to be moved to District 10. Things go very wrong, especially in the case of the leader of the removal group. I'd tell you what happens to him, but it's something you need to find out for yourself. Let's just say that he finds it very unpleasant, and it leads to his finding out some unsavory secrets as the movie gradually drops the pseudo-doc style and makes the transition to an all-out balls-to-the-wall action movie in the final act.
There are some nice surprises as things advance, and while some things are left unexplained, maybe we'll find out the answers in the sequel that's clearly being telegraphed in the final scenes. It should be interesting, and I'll be on board.