For a start, you either love Simon Pegg and Nick Frost or you don't. This is the third film in the planned trilogy of films, coming after Shaun Of The Dead and Hot Fuzz. And this is very, very funny.
Basic premise, without spoiling anything, the two main leads are science fiction 'geeks' who have gone to a convention in America. They have seemingly been interested in Aliens (talking and even writing related comic books) for years and years. While driving across country in a rented mobile home, they witness a car mysteriously flip and crash, for no apparent reason.
They then meet Paul - an alien, who is on the run from a Government camp intent on conducting experiments on him - and just wants to go home.
If I give you more details on the story, I will spoil it and I don't want to do that. Suffice to say, they encounter obstacles along the way, pick up an extra passenger by accident - who adds a lot of comedy to the film - and are being chased by rogue police officers. Let's face it, in every film like this, you need the bad people and the element of threat.
If you love these two, and find their humour hilarious, as I do, then my advice is to get this film. You will love it! And it's quite cheap now too, so one for the collection!