British sci-fi nerds Graeme Willy (Simon Pegg) and Clive Gollings (Nicholas Frost) are in the US on the trip of a lifetime. Heading first to geek Mecca, sci-fi convention Comic Con, the two pals plan to round off their holiday with a road trip through a number of famous UFO hotspots. The die-hard dweebs get more than they bargained for, however, when a car crashes in front of their RV and a little green alien emerges from the wreckage.
Foul-mouthed and fun-loving, Paul (voice of Seth Rogen) is being pursued by government agents after escaping from Area 51 on a mission to meet a rescue party of his fellow ETs. Despite Clive's reservations, he and Graeme agree to assist the diminutive alien, and the duo are soon caught up in a crazy cross-country adventure that will see them deal with religious bigots, unhinged Feds, ruthless baddie 'The Big Guy' (Sigourney Weaver) and Steven Spielberg.