Much of science fiction explores the moment of first contact - what will people do when the aliens land? From H.G. Wells' pioneering The War of the Worlds to District 9, Invasion deals with our fears of alien invasions of earth. David Tennant explains the appeal of Doctor Who's Daleks and Cybermen while John Carpenter and Chris Carter explore the rich appeal of the paranoia fuelled by hidden aliens with The Thing and The X-Files. The episode also asks, what if the monsters were our own creation?