Plot and some spoilers from Wikipedia
The film opens in 1947, with a dog scratching to be let outside, where the sky is covered in eerie lights. When his owner, a young girl lets it out, the dog, named Paul, is crushed by a crash landing spaceship. The girl pulls an alien from the wreckage, whom she names Paul, and he is taken away by the Government and held prisoner for 60 years. He eventually decides to escape from his holding place, Area 51.
Two British comic book nerds named Graeme Willy (Simon Pegg) and Clive Gollings (Nick Frost) have travelled to America for a huge comic book convention and to visit all the sites of major extra terrestrial importance. On the way to their second site, they stop off at a café, where two soldiers confront them. As they hurriedly leave, they reverse into the soldiers’ car, leaving a dent. At the next site, they see a car blasting towards them and, believing it to be the soldiers, they speed off in their RV. Eventually, the car catches up, and they realize that it is not the soldiers. As it overtakes, it speeds off the road, rolls several times, and comes to a halt in a field. The two shocked men get out to investigate. When they realize that the vehicle is empty and begin phoning the emergency services, they hear a voice telling them not to. They turn around, and a little green alien comes into view, smoking a cigar, and introduces himself as Paul. Clive faints, and Paul explains to Graeme that he is on the run and needs their help. Graeme agrees to let him come. When Clive wakes up, he is not happy about the idea, but is eventually brought around.
Along the way, they are forced to take with them a Christian named Ruth (Kristen Wiig), who, until Paul shows her his memories, believes that there is nothing outside Earth and that God created humans. When he shatters her faith in Christianity, she suddenly becomes eager to ‘sin’, which she was not allowed to do before by her controlling father, who pursues them. She initially doesn’t trust Paul, but to prove himself trustworthy, he heals her eye, as she has been blind in it since the age of four. The whole way, three secret agents, apparently eager to kill Paul under orders from a woman over the radio, have tailed them.
After a while, Paul decides to return to the girl whose dog he crashed his ship on, who is now an old woman, Tara Walton (Blythe Danner). She turns her gas cooker on but never gets round to doing anything with it, because they are interrupted by the agents. As the motley crew drives off with Paul, one of the agents shoots at them, and the gas ignites, destroying the house. Another agent catches up with them but drives off a bridge and is also killed.
By this time, Paul has revealed that his people are ready to pick him up, and he is heading to the rendezvous. When they get there, they set off a signal and wait. Eventually, eerie orange lights show up over the surrounding trees, and everyone believes that it is Paul’s race. However, it is an army helicopter, with the ‘Big Guy’ (Sigourney Weaver) on board, who is actually the woman commanding the three agents. As she and three troops backing her up move to shoot Paul, the third agent, Lorenzo Zoil (Jason Bateman), who turns out to be working for Paul, bursts out of the trees and takes out the men, but is shot in the shoulder. Tara knocks out the ‘big guy’, but Ruth’s father appears with a shotgun and shoots Graeme dead. Paul heals him, and the ‘big guy’ wakes up, but is crushed by an alien ship. Paul leaves in the ship, and takes Tara with him
Trailer 1
Trailer 2