When Earth astronaut Capt. Chuck Baker arrives on Planet 51 -- a world reminiscent of American suburbia circa 1950 -- he tries to avoid capture, recover his spaceship and make it home safely, all with the help of an empathetic little green being.

With a house to sell and no time to be there in person, overworked TV producer Chris Ashworth is forced to leave his incompetent flatmate Bob to show real estate agents around the property. Knowing that Bob is not the most attentive flatmate, Chris tells him to show anyone that calls inside. When Chris finally returns home he finds that Bob has taken him at his word, literally. With four estate agents, some Jehovah's Witnesses, an incontinent American, a corpse and two armed gangsters, Chris's world may look like its falling in, but there's worse to come.