In Santa Mira, California, Dr. Miles Bennell (Kevin McCarthy) is baffled when all his patients come to him with the same complaint: their loved ones seem to have been replaced by emotionless impostors. Despite others' dismissive denials, Dr. Bennell, his former girlfriend Becky (Dana Wynter), and his friend Jack (King Donovan) soon discover that the patients' suspicions are true: an alien species of human duplicates, grown from plant-like pods, is taking over the small town.