The Body Snatchers

Dr Miles Bennell has lived there all his life. But one day Miles sees a patient who claims her Uncle isn't himself. He's a different person, despite being identical in every way except one: he is only pretending to have emotions.
Miles dismisses this as delusions and refers her to a psychiatrist.
Then he finds the pods. Giant seed pods, filled with a strange, grey substance. A strange grey substance that can slowly, slowly, become a perfect replica of a person.