The aliens have been with us for 20 years already at the start of South African director Neill Blomkamp’s fast and furiously inventive District 9, their huddled masses long ago extracted from their broken-down mothership and deposited in the titular housing slum on the outskirts of Johannesburg. As the movie begins, a wave of violent alien-humans has prompted the good people of Jo’burg to crave even greater distance from their unwanted neighbors, and a forced relocation of all alien residents (disparagingly referred to as “prawns”) to a Guantánamo-style tent city known as District 10 has become law. Enter Multi-National United, a smarmy private military contractor that places the relocation in the hands of one Wikus van der Merwe (Sharlto Copley), a not-very-bright corporate lackey who also happens to be married to the boss’s daughter. Soon everything goes haywire, with the oppressor getting a crash course in what it feels like to be the oppressed. District 9 is never better than in its first 45 minutes, as Blomkamp maps out the film’s social and economic realities—alien language, graffiti, black-market goods—via a grab bag of news reports, corporate videos, and CCTV cameras. But even in the more conventional second half, Blomkamp puts things across with terrific verve, using action and computer effects to enhance rather than trump story and character.