In San Francisco, deputy inspector Matthew Bennell and microbiologist Elizabeth Driscoll are friends and colleagues in the food division of the county's Department of Public Health. One day, Elizabeth notices that her live-in dentist boyfriend, Dr. Geoffrey Howell, is acting strangely, almost like his being has been taken over by someone else. She begins to see this strange behavior in many others, and discovers Geoffrey meeting with like-behaving people she doesn't know, leading her to believe that there is some sort of conspiracy happening. She tries to convince Matthew that something is going on, he looking for some rational explanation into what he also sees, in the process consulting his celebrity psychiatrist friend, Dr. David Kibner, who attributes such behavior to something best treated in couples' counseling. Matthew is forced to admit that something other worldly is happening when he sees evidence of such provided to him by his struggling writer friend Jack Bellicec and Jack's mud bath owner wife, Nancy Bellicec. Matthew not only has to try now to convince others of what he saw but also figure out what exactly is happening to be able to stop it. If Matthew and Elizabeth are unable to do either, the consequence may be the human race being taken over by these other worldly beings. In the process, they also have to learn who they can or cannot trust, as some people may already have changed like Geoffrey without them knowing.—Huggo