Plot: Dominic and leading writers and film-makers look at science fiction's obsession with alien invasion, from all-out assault to sinister hidden threats, and how it has reflected real-life anxieties - whether they be the challenge to Victorian imperial power of HG Wells's War of the Worlds, the Cold War-era paranoia of Invasion of the Body Snatchers or more recent concerns about racism and immigration in District 9. And we celebrate the most famous alien invaders of all - the Daleks