Held by the police as a raving lunatic, Dr. Miles Bennell recounts to a psychiatrist the events that have turned his life upside down. He returned to his small town the previous Thursday, having been called back from a medical conference by his nurse, who was being flooded with patients. He arrives to find that most have canceled their appointments, but the few cases he does have all have the same story: someone close to them is acting strangely as if they had been replaced. Consulting some of his colleagues, he finds that these types of reports have been coming in all week and they conclude it must be some type of mass hysteria. However, when his friends Jack and Teddy Belicec show him a partly formed body they have uncovered in their home, he begins to realize that there may be some truth to the wild stories he has been hearing.