Consumer note: to catch all the in-jokes in this movie, it helps to have a decent familiarity with a whole pile of movie and TV science fiction over the past few decades: the collected movie oeuvres of Spielberg, Cameron, Lucas, Zemeckis, Schwarzenegger and (of course) all things Star Trek would be a good start. No doubt part of the reason I liked the movie so much is that the filmmakers seem to be the same genus of geeks.
Also, it's a pretty rare movie that jokes about the Fermi Paradox.
Simon Pegg and Nick Frost play Graeme and Clive, two geeky but not-too-bright Brits, in America. Starting at ComicCon in San Diego, they plan to rent an RV to check out all the SF hotspots: Area 51, Roswell, etc. By utter chance they encounter Paul, an alien on the lam from the government functionaries that have kept him hidden and imprisoned for years. They decide to help him make a planned rendezvous with the alien rescue party he's summoned.
What follows is an entertaining road trip. While eluding government agents, they also encounter a lot of colorful characters, most notably Ruth, played by Kristen Wiig. She's a fundamentalist Christian, but Paul's presence causes her to drop her beliefs like a hot phaser.
The movie's rated R for pervasive crude language (especially Ruth's: this is a go-to movie if you've always wanted to see Kristen Wiig talk dirty). And Christians without a sense of humor might be offended. (John Nolte, for example: "just more of the strident, boorish evangelical atheism we’re seeing from our entertainment overlords these days") I tend to be more easygoing about these things.