This extraordinary science fiction film follows the fate of alien refugees forced to live in Soweto-like squalor in Johannesburg where their space ship, still hovering over the city, brought them. In a telling reference to apartheid, the government forces them to move, triggering violent opposition. Superb special effects bring the despised aliens, called “prawns” by the locals because of their giant crustacean-like appearance, to vivid life, but the power of co-writer-director Neill Blomkamp’s drama is as much in what it evokes as what it depicts.