One of my favourite films is the original 1956 black and white version of Invasion of the Body Snatchers. The film tells of strange alien "pods" that produce simulcra of people as they are sleeping. The simulcra are emotionless versions of the people they have replaced. The film has been interpreted as a commentary on both the insidious advance of communism and McCarthyism, which proves that people will see what they want to see.
The film is not without flaws as the studio didn't believe that the original downbeat ending would be popular. If you want to see the film as it was intended, you should stop watching as the hero, Miles, runs on to the freeway and frantically shouts to the passing motorists, "They're here already! You're next! You're next!" If you carry on watching you will see the dreadful tacked on ending where the authorities suddenly believe Miles about the alien invaders and mobilise the police to pick up the pods.
Miles tries to warn the motorists
There have been a couple of other versions of the film: the decent 1978 remake with Donald Sutherland and the much less successful 2007 film "The Invasion" starring Nicole Kidman.