Invasion of the Body Snatchers (1956).

Something evil has taken possession of the small town of Santa Mira, California. Hysterical people accuse their loved ones of being emotionless impostors; of not being themselves. At first, Dr. Miles Bennell (Kevin McCarthy) tries to convince them they're wrong…but they’re not. Plant-like extraterrestrials have invaded Earth, replicating the villagers in giant seed "pods" and taking possession of their souls while they sleep. Soon the entire town is overwhelmed by the inhuman horror, but it won't stop there. In a terrifying race for his life, Dr. Bennell escapes to warn the world of the deadly invasion of the pod people! Remade in both 1978 and 1997, this chilling combination of extraterrestrial terror and anti-conformity paranoia is considered one of the great cult classics of the genre.