Aliens from Outer Space are slowly switching places with real humans -- one of the first being a young man about to get married...

John Putnam is a writer and an amateur stargazer with a new home out in the beautiful Arizona desert, which he enjoys with Ellen Fields, his girlfriend and a local schoolteacher. John is not trusted by the people of the small town near where he lives, certainly not by Sheriff Matt Warren, who feels protective of Ellen, and perhaps something more. One night, John and Ellen see a meteor crash in the desert. John drags his friend, Pete, out of bed to take him over to the crash site in his helicopter. Once there, John climbs down into the crater. Unfortunately, he does so alone, as Pete and Ellen wait for him. John is the only one who sees the spaceship before a landslide covers it. And John is the only one who catches a glimpse of the hideous thing inside. At first John's story seems mad, until some of the townsfolk begin acting strange - as if they aren't really who they seem to be.

Scientists and American Air Force officials fend off a blood-thirsty alien organism while at a remote arctic outpost.

Dr Miles Bennell returns his small town practice to find several of his patients suffering the paranoid delusion that their friends or relatives are impostors. He is initially skeptical, especially when the alleged dopplegängers are able to answer detailed questions about their victim's lives, but he is eventually persuaded that something odd has happened and determines to find out what is causing this phenomenon. This film can be seen as a paranoid 1950s warning against those Damn Commies or, conversely, as a metaphor for the tyranny of McCarthyism (or the totalitarian system of Your Choice) and has a pro- and epilogue that was forced upon Siegel by the studio to lighten the tone.

The sensationalist reporter Michelle Fox presents the TV show Weird World, with phony matters about UFOS and aliens. When she hears about Cat, a young woman that claims that have been abducted with her boyfriend and become pregnant by aliens, she convinces her chief to travel with a team to the remote Welsh island of Scalled to interview Cat. She invites the cameraman Ricky Anderson with his sound technician partner; the nerd expert in "ufology" and "ley lines" Gavin Gorman; the actress Candy Vixen and an obscure gay actor to prepare the matter. They get a van and wait for the low tide to reach the island, and when they find evidences that aliens are really landed in the location, the ambitious Michelle decides to film her way to fame and wealth.