In Norrisville, Bill Farrell leaves his bachelor party on the eve of his marriage with Marge Bradley. He is abducted by an alien that takes his shape and marries Marge on the next day. Marge feels something strange with Bill and one year later she realizes that he is a totally different man. One day, Marge follows Bill and he goes to the woods; she finds that he is an alien and sees his spacecraft. She tries to tell to Washington and to the FBI, but the aliens have dominated key people in town that do not allow any sort of communication with the exterior world. What is the intention of the alien invasion?
Aliens from Outer Space are slowly switching places with real humans -- one of the first being a young man about to get married. Slowly, his new wife realizes something is wrong, and her suspicions are confirmed when her husband's odd behaviour begins to show up in other townspeople.
Bill Farrell marries lovely Marge Bradley, but almost from the start he is acting strange. For one thing he doesn't drink any alcohol and even more peculiar, he almost acts like he doesn't have any emotions. A year into their marriage, Marge still feels that there is something strange about him. The thing that really disturbs her is the fact that after a year of marriage, she can't seem to get pregnant. One night she notices Bill leaving the house going for a walk and follows him. She then sees him meeting with the other aliens and soon realizes that it is not her fiancée. The alien "Bill" soon explains that the planet he comes from was destroyed and that he and the other males were able to escape. However, the females of their planet were unable to escape and were killed in the disaster. He then explains that the reason they are taking over the bodies of the Earth men so that they can mate with the women. A horrified Marge then tries to warn everyone of the alien threat.
Aliens arrive on Earth to possess the bodies of humans. One of their first victims is a young man, whose new wife soon realizes something is wrong with him.
- The movie opens with title and credits shown with the Earth over a star field background. We zoom in on the Earth, and then fade to a pastoral scene of a park with a lake in the small town of Norrisville. A car drives up a two-lane road and pulls into the parking lot of the local bar and restaurant, King's. Two men exit the car and approach the entrance and we hear music coming from inside. They see a man and woman necking in a car. Five drunk men sit at a table. It is a bachelor party of sorts for Bill Farrell (Tom Tryon). He is getting married the next morning, so he leaves early. Sitting next to Bill on his right is his friend Sam Benson (Alan Dexter) and on list left is Mac Brody (Ty Hardin as Ty Hungerford). He reminds Bill that seeing his fiancé before the wedding is bad luck. Harry Phillips (Robert Ivers) jokes that Bill is such a nice guy, It's a shame it has to happen to him. Bill gets in his car and drives off to see Marge. He drives along the lake and sees a body in the middle of the road and stops. He gets out of the car, sure he hit a person, but there is nothing there. He observes, "I didn't think I had that much to drink." A strange, glowing arm with three pointed fingers touches his shoulder. Bill turns around and recoils at the sight of a very strange humanoid alien (an uncredited Charles Gemora). Bill sinks down along the front fender of the car to the ground and is enveloped in a dark, billowing cloud. When the cloud disappears, so does Bill.
It is morning and we are in the wedding chapel. Mrs. Bradley (an uncredited Mary Treen), the mother of the bride enters the room with flowers and humming the Wedding March, much to her daughter's annoyance. Marge (Gloria Talbott) is anxious as Bill has not arrived for the wedding. Bill walks in and apologizes for being late. After the ceremony the couple exits the Wee Kirk Wedding Chapel and gets in the car. The well-wishers throw rice. Marge throws the bouquet and Helen Rhodes (Jean Carson) catches it. She wants to marry Sam and intends to soon, if she can get him to the altar. As Bill and Marge drive past the lake the alien watches them. Bill almost hits another car on the road. He "forgot" to turn on his headlights. When Marge innocently asks about the mishap, he snaps at her. They arrive at their honeymoon hotel near the ocean. Bill forgets to open the door of the car for his new wife. She tells Bill, "I know you've been absent-minded lately, but you can't have a honeymoon without a bride." They share glasses of champagne. A storm brews outside as the couple prepare for bed. Marge is starting to think something is strange with her new husband. Marge goes inside and Bill stays outside to watch the storm. A flash of lightning reveals it isn't Bill, but a hybrid of Bill and the alien he encountered on the highway.
One year later Marge composes a letter to her mother. She complains that she is both frightened and bewildered. "Bill isn't the man I fell in love with. He's almost a stranger." Marge decides not to send the letter. Sam and Ted Hanks (Chuck Wassal) are sitting in a bar and talk about not seeing Bill Farrell any more. Sam gets his coat and tells the bartender, Max Grady (Max 'Slapsie Maxie' Rosenbloom) that he is a home-wrecker. Sam starts to walk home, but he feels ill and wanders into an alley to puke. He hears a strange noise and turns to investigate. He falls backwards in shock and is enveloped in a roiling black cloud, and like Bill, when the cloud disappears, so does Sam.
Marge looks at an X-Ray of herself and complains, "That doesn't look like me at all." She is meeting with Dr. Wayne (Ken Lynch). She saw the doctor to find out why she hasn't been able to get pregnant. The doctor admits he and his wife, Doris, took five years to have their first child. Before Marge gets out the door, Dr. Wayne asks, "Why don't you have Bill come in and see me." Marge stopped at a pet store and placed the cage in her car. A car honks its horn and pulls up behind Marge. Helen Rhodes pulls up with Sam Benson. She announces she and Sam are to be married. Marge drives home and gets the cage out of the car. Bill is outside and calls to her, spoiling the surprise. She pulls the cover off the cage and the dog growls at Bill. The dog snaps at Bill when he puts his hand near the cage. Bill tells Marge that perhaps dogs don't like him, but she reminds him he's had dogs all his life. Bill tells Marge to put the puppy down in the basement. Later that evening Bill goes into the basement. The dog is just as aggressive. Bill reaches out to the dog and kills it. Marge runs to the basement to check on the noise. Bill tells Marge the puppy is dead. When Marge asks how, Bill lies and tells her, "Well, he must have strangled himself. I think his collar was too tight." Marge tells Bill she saw Dr. Wayne today. She tells him she wants children and Bill should see the doctor. The doorbell rings. Marge answers it. It is Sam and he wants to see Bill. They talk briefly, and Marge excuses herself and walks upstairs. Alone, Sam reveals himself to Bill. He asks Bill, "Did you make many mistakes at first?" Sam admits he likes the pleasures humans enjoy, despite the stupidity, bad design and short life span. Sam finally gets to the point, "They've improved the methane reservoirs in these bodies. You're due to report to the ship tonight." Sam reveals his alien/human hybrid face.
A police car pulls up a street and stops. Officer Schultz (Jack Orrison) asks his partner, Officer Frank Swanson (Peter Baldwin) if everything is ok. They see a drunk in the alley and go to investigate. Swanson hits Schultz, and then he and the "drunk" carry him out of sight up against a wall. The cloud envelopes Officer Schultz. It is clear now that many of the men in town have been abducted. After checking on his wife, Bill leave the house. He is unaware that Marge has seen him leave. She grabs a coat and slips it on over her nightgown. She follows her husband. The sound of a screeching cat draws her attention. Bill wanders back to the ship with Marge closely behind. She follows the sounds of foot falls in dry brush and spots Bill. She sees a black vapor exit her husband and an alien materializes. The glowing alien walks into a spaceship. Bill remains standing and Marge walks over to him. He is unresponsive and when she touches him, he falls backward, his eyes staring into space. Marge runs. She wakes in the gutter in town. She is drawn to the noise in the bar. She tells Max Grady the Bartender she just saw a monster. Sitting and leering is Weldon (James Anderson) and he instantly takes an interest in Marge. He thinks she is drunk or having the DTs and comments to Max, "Funny thing. She didn't look like a lush." Francine (Valerie Allen), the town hooker walks over and talks to Weldon, but he expresses no interest. Marge stops two policemen (Swanson and Schultz) in their car and asks to see Chief Collins (John Eldredge). Collins offers her a cookie and pours her a cup of coffee in his office. He listens sympathetically and assures her she is sane. But he doesn't believe in the aliens invading the Earth. He convinces Marge to go home and pretend everything is fine. He asks Swanson to take Marge home. Collins walks over to the window. A flash of lightning reveals his true nature. At home, Marge walks slowly into the living room. Bill is there, in the dark, and turns the lamp on. They walk upstairs.
It is Sam's wedding day and the rehearsal is underway. Bill is acting as Best Man, and Marge is the Matron of Honor. Marge asks to speak to Helen. Marge begs Helen not to marry Sam, not today, but sometime later. Marge is not specific, and Helen ignores her suggestion. Bill walks in to tell Helen she is needed outside. Weldon has Marge Farrell's address and walks over to her house. Inside, Marge is looking for a lighter. She asks Bill why he isn't drinking. Bill comments that Marge has changed over the last few weeks. She avoids his presence, and his touch. Bill looks out the window and see Weldon. Suspecting something, he mentally summons Officers Schultz and Swanson to the house. They confront Weldon and arrest him. Weldon admits he's been sniffing around Marge. He pulls out a gun and shoots Swanson point blank three times. Schultz cold cocks Weldon. He asks his partner if Weldon is any use to their cause. Frank shakes his head, no, so Schultz kills the man with his service revolver. The shot wakes Marge. She and Bill talk, he admits he's learning. Bill leaves the house. Francine is on the move at the bar. Bill, Sam and Harry sit and stare at their drinks while they talk. They discuss pretending to be human. Harry thinks they are disgusting, but Sam admits he like them. Sam reveals the ultimate goal of their visit to Earth, "Look, Harry, our scientists are working on a way right now to mutate human female chromosomes so we can have children with them." Max notices the three men haven't touched their liquor and confronts them. He hits Bill twice, but Bill is unfazed and responds by putting his hands in his pockets. Max is surprised and a little scared. Max stares at his bloodied knuckles. Francine decides to leave the bar. She adjusts her nylons, then saunters down the street. She sees a hooded figure staring into a toy shop display window. She tries to attract a potential customer by asking for the time. The hooded figure refuses to answer Francine. It finally turns to face her and she is shocked. She runs away screaming. The figure points a weapon at her, fires, and she disappears into a glowing mass.
Several couples are enjoying a picnic in the park. Ted Hanks teases his wife, Caroline (an uncredited Darlene Fields) about her throwing arm. Helen and Sam are out on the lake in a row boat. Sam falls overboard, and Ted comments, "Oh, he can swim like a fish." Marge notes that he is not swimming. Ted dives in to rescue his friend, while the rest of the men stand and stare. Helen dives in after her new husband. On shore, Doctor Wayne gives Sam oxygen, but instead of reviving him, it seems to kill the man. Helen spends the night with the Farrells and Marge checks on her before walking downstairs. She goes back to see Capt. Collins. Collins warns Marge about her delusions. Marge tries to call Washington, D.C. on a pay phone, but can't get through. She walks over to the Western Union office and talks to Mr. Potter (an uncredited Tony Di Milo). She fills out a telegram form to the FBI, pays the fee and walks away. She sees, through the window, that Potter tears up the form. She tries to drive out of town but is stopped at a road block. Swanson claims the road was washed out.
At home, Marge sits on the couch in the dark. When Bills asks her, "How about some light?" She tartly replies, "You don't need any." She finally confronts the Alien Bill and tells him, "I know you're not Bill. You're some thing that crept into Bill's body. Something that can't even breathe the same air we do. Sam died because oxygen was forced into his lungs." Bill tells Marge he comes from a planet in the Andromeda Constellation. Their sun became unstable so they built a fleet of space ships to get away and that all their females died. He reveals their plans to breed with human females.
Marge reveals all to Dr. Wayne. He believes Marge and comes up with a solution for finding human men. He needs help. Ted walks over to announce his wife delivered twins. Dr. Wayne recruits the men waiting in the maternity ward. Marge returns home. Bill is waiting in the living room. The posse of men arrive on the road to the field where the spaceship is located. One man, Charles Mason (Steve London) brings his hunting dogs. They take the path and fan out to search. Bills calls Chief Collins on the phone to inquire about his wife. Bill easily knocks down the door to their bedroom. The posse approaches the ship. A strange detector, a series of pipes and cylinders, scans the area. As Bill shakes Marge, demanding to know who she spoke to, he senses the approaching danger to his ship. In fact, all the alien imposters know and respond to the danger. Bill tells Marge, "So your friends are attacking our ship?" The ship opens and a couple of aliens exit. Bill gets into the police car and returns to his ship. Dr. Wayne and Mason fire at the aliens, but bullets enter, and the wounds instantly seal up. The posse retreats, but Mason releases his dogs. One of the dogs pulls the aliens breathing tube off. It bleeds from the wound, and the creature falls to the ground. The other dog attacks an alien in a similar fashion and the alien is also killed. The men return and stare at the alien body that disintegrates into a pile of gel-like goo. A small explosion destroys their ray gun. The posse enters the ship. Suspended above the floor are the real humans. They are attached by wires to small machines on the floor. Collins, Bill, Sam, and the two police officers are along one wall. Five more men are on another wall, and they include Harry and Mac. Dr. Wayne concludes, "Electrical impulses from the real human bodies must give the monsters form and shape, even memories." Dr. Wayne disconnects Officer Schultz and his corresponding alien falls to the ground and turns into the gel-like goo in clothing. As Dr. Wayne disconnects more humans, more aliens are stopped. Alien Bill realizes the mission has failed. Alien Capt. Collins reports on a battery shaped device, "Earth mission has failed. They're alerted and dangerous. Suggest continuing on to another galaxy. Total personnel lost." With his human plug pulled, Alien Collins collapses in his office. He finishes his report, "Destroy scout ship." He then collapses and turns into the gel-like goo. As the posse removes more men, Marge approaches the Alien Bill. He tells Marge, "Your people have won. That makes you happy, doesn't it?" Bill's plug is pulled and his alien counterpart collapses and writhes in pain. His body turns into a Jello-like goo. The real Bill calls to Marge. He smiles and they embrace. The ship explodes, and Marge and Bill embrace. We close with a scene of the Earth in space and a fleet of flying saucers traveling away.