In California, at the request of Dr. Harvey Bassett, psychologist Dr. Hill is brought in to confer about Dr. Miles Bennell, who has been admitted to the hospital as a mental patient. Although highly agitated, Miles convinces the doctors to listen to his story, which began a few days before, when Miles left a medical conference early, at the urgent request of Sally, his nurse: Although the small town of Santa Mira seems unchanged when he arrives, Sally reports that his waiting room is filled with patients. Driving to his office, Miles narrowly escapes hitting young Jimmy Grimaldi, who darts into the street. Grandma Grimaldi explains to Miles that Jimmy is afraid to go to school and, when Miles notices that her successful vegetable stand is closed, she says her husband lost interest in it. At the office, Miles finds few patients waiting, but, later, Becky Driscoll, Miles’s former girl friend, comes to consult him about her cousin, Wilma. Becky, who has until recently been living in England, explains that Wilma insists someone is impersonating their uncle, Ira. After Miles offers to see Wilma, he and Becky chat and discover that they have both recently divorced. As Miles walks Becky outside, he speaks with a policeman who asked Sally for an appointment but now says it is unnecessary. Near the end of the day, Grandma brings in Jimmy, who says someone is pretending to be his mother. After talking to Jimmy, Miles, who is intrigued by the similarity of his and Wilma’s claims, proceeds to Wilma's house and determines that Ira is not an imposter. Wilma disagrees, saying that the imposter looks like Ira and has his memories, but lacks emotion, and Miles tactfully refers her to a psychologist, Dr. Danny Kauffman. That evening, Miles and Becky go out together and encounter Kauffman and another doctor, both of whom believe that mass hysteria has stricken the townspeople. Later, Miles and Becky find that a once-popular restaurant is empty of customers and has been for the last two weeks, according to the proprietor. An emergency call from Jack and "Teddy" Belicec prompts Miles and Becky to visit them. At their home, the couple shows them a mysterious, half-formed body, without facial lines or fingerprints, found lying inert on their billiard table. When Teddy points out that the height and build are similar to Jack’s, her husband is startled into dropping a drinking glass that cuts his hand. Miles suggests they watch the body until morning. He then takes Becky home, where he checks out an unsettling noise caused by her father working in the basement. During the night, Jack dozes as Teddy notices that the body has a cut on its hand in the same place as Jack’s. Screaming, she awakens Jack and they drive to Miles’s house. Miles calls Kauffman and then, acting on a premonition, rushes to Becky’s house, breaks into the basement and discovers a double of her forming there. He rescues the real sleeping Becky and takes her to his house. Accompanied by the skeptical Kauffman, Miles and Jack return to the Belicecs and discover that the body is gone. They also find Becky’s double missing. When Becky's father, having heard them in the basement, calls the police station, a policeman, Nick, responds to check out the house. When Miles tells him about the mysterious body at Jack's house, Nick says that it belonged to a murder victim later found burning on a haystack. The next morning, Wilma tells Miles that she feels better and withdraws her allegations about Ira. Grandma and Jimmy come to Miles's office and inform him that the boy is back to normal, prompting Miles to wonder why the strange ailments have mysteriously disappeared. While making dinner at Miles's house that night, Miles, Becky and the Belicecs discover four pods growing in the greenhouse. Horrified, they watch as one body pops out of a foaming pod. Miles guesses that the pods create a double of a person while he or she sleeps, after which the original body is either destroyed or disintegrates. It then occurs to Becky that her father has acted strangely since her return and is one of the pod people. Miles attempts to call the FBI in Los Angeles, but an emotionless telephone operator claims that she cannot get through. Increasingly frantic, Miles asks her to try Sacramento. While waiting, Miles discusses with his friends whether the pods might be caused by an atomic reaction or alien organism, and how the new bodies take over the person’s mind. As they wait, one of the pods forms a double of Teddy. Sensing immediate danger, Miles asks Jack to take the women to safety, but Becky insists on staying with him. Miles kills the bodies with a pitchfork and leaves with her. At a gas station, the attendant slips two pods into his trunk, which Miles finds and burns later. Hoping to rescue Sally, Miles and Becky drive to her home, where Miles learns that she, too, has been transformed. Nick, whom he now realizes has been transformed all along, corners him, but Miles escapes with Becky, after which Nick alerts the police to search for them. Abandoning the car, Miles and Becky hide in his office, planning to rendezvous with the Belicecs there. After giving himself and Becky a shot of a drug stimulant to keep them awake, Miles says that, in his work he often sees people slowly grow callous and lose their humanity, and is troubled how they do not seem to mind. The next morning, Miles and Becky watch through the window as farmers distribute to their emotionless neighbors pods that are destined for other towns. Then Jack and Kauffman appear, both transformed, and place pods in the office for Miles and Becky. Kauffman explains to them that the solution to humanity’s problems has come from space seeds. The pods from these seeds reproduce an exact likeness of any form and painlessly absorb its mind, so that the being awakens into an “untroubled world.” Although Becky and Miles argue that they prefer to have love, Kauffman points out that life is simpler without it. After tricking the men, Miles injects them with sedatives, then flees with Becky. On the street, they hide their emotions, pretending to have changed, but Becky shrieks when a truck almost hits a dog, thus giving them away. Pursued by the townspeople, they run up a hill and into an old mining tunnel, where, exhausted but fighting sleep, they wait until nightfall. The sound of beautiful music coming from outside the cave gives them hope that others have survived. Miles leaves Becky to investigate it and learns that the music is coming from a car radio. When he returns, Miles kisses Becky but, to his great shock, realizes that she has been changed, having fallen asleep in his brief absence. Panicking, Miles runs toward the highway after Becky alerts the others, but the townspeople let Miles escape, presuming that no one will believe him. Through the dense, slowly-moving traffic Miles runs, screaming warnings, but the drivers think he is either drunk or crazy. When seed pods fall out of a truck, Miles yells, “They’re here already! You’re next!” At the hospital, Miles ends his tale, aware that the doctors will not believe him. Then, an ambulance delivers casualties from a highway accident, in which victims were buried under large pods that fell from a truck. Upon hearing that the truck originated in Santa Mira, the doctors call the FBI and police, to the relief of the exhausted Miles.