Something is amiss in the sleepy California hamlet of Santa Mira. Initially, Dr. Miles Bennell (Kevin McCarthy) dismisses reports from the citizenry accusing friends and family of behaving like automatons.
But the doctor soon makes a startling discovery: Extraterrestrials capable of replicating humans and assuming their identities have invaded Santa Mira. It's up to Bennell to sound the alarm and almost single-handedly battle the alien forces.