Pegg and Frost are two U.K. sci-fi nuts in the states to attend Comic Con in San Diego, then tour Area 51 and Roswell, N.M., in a Winnebago.
The big gag - in the desert, they meet a real alien (animated, voiced by Seth Rogen), and end up helping him escape from the feds (Jason Bateman, Sigourney Weaver).
The other big gag - "Paul" pokes fun at the grown-man geek fandom of its two leads (neither has slept with a girl not dressed as a Klingon), then weaves into its plot a billion in-joke references for sci-fi geeks and movie buffs.
Some are obscure ("Lorenzo's Oil"?), some obvious - one "E.T." riff has Paul posing as a stuffed toy while hiding from the federal agents in a sci-fi themed souvenir shop.
Not so original are the repeated, easy-target jokes about fundamentalists - extraterrestrial Paul mocks the creationist beliefs of the Christian abductee (Kristen Wiig) who's also along for the ride.
Paul's smug lectures about superstitious thinking might sound better if they were not coming from an animated alien waiting to board a flying saucer.
Themes of faith and science fiction were handled more delicately in the obscure 1980 comedy "UFOria," worth watching if you can find it.