In San Francisco, public health inspector Matthew Bennell is intrigued when a co-worker and good friend, Elizabeth Driscoll, tells him that her husband has suddenly changed. He suggests they speak to his friend, psychiatrist David Kibner, who confirms that several of his patients are claiming that their husbands or wives are not who they seem to be. He puts it down to mass hysteria until two other of Matthew's friends, Jack and Nancy Bellicec, discover a partly formed corpse. They soon discover that humans are being replaced with exact copies who are emotionless and are moving forward apparently unstoppable, in their quest to replace everyone.
