Stop. What follows is a spoiler filled review of Duncan Jones' Moon
There is no twist in Moon. Well, not in the traditional sense anyway. There is, though, a plot development about twenty minutes in that, if revealed beforehand, would rob the film of some of it's secrets. It's almost impossible to discuss how well the film works without addressing, in some detail, that change in direction.
Sam Rockwell plays Sam Bell. An astronaut at the end of a three year assignment on an isolated Moon base harvesting Helium 3, a new, very efficient energy source, and sending it back to Earth. Even though he receives messages from his wife and two year old daughter the last three years of solitude have taken their tole on him. He is lonely, disconnected and suffering hallucinations.
After an accident on the surface, Sam wakes up in the base infirmary and journeys back out to the crash site where he finds (here be spoilers) himself.