Dr Miles Bennell returns his small town practice to find several of his patients suffering the paranoid delusion that their friends or relatives are impostors. He is initially skeptical, especially when the alleged dopplegängers are able to answer detailed questions about their victim's lives, but he is eventually persuaded that something odd has happened and determines to find out what is causing this phenomenon. This film can be seen as a paranoid 1950s warning against those Damn Commies or, conversely, as a metaphor for the tyranny of McCarthyism (or the totalitarian system of Your Choice) and has a pro- and epilogue that was forced upon Siegel by the studio to lighten the tone.