In the year 3000, man is no match for the Psychlo's, a greedy, manipulative race of aliens on a quest for ultimate profit. Led by the seductive and powerful Terl, the Psychlo's are stripping Earth clean of its natural resources, using the broken remnants of humanity as slaves. What is left of the human race has descended into a near primitive state, believing the invaders to be demons and technology to be evil. After humanity has all but given up any hope of freeing themselves from alien oppression, a young man named Tyler decides to leave his desolate home high in the Rocky Mountains to discover the truth, whereupon he is captured and enslaved. It is then that he decides to fight back, leading his fellow man in one final struggle for freedom.

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.

When one saucer of an invasion force has engine trouble, it lands on Earth. It happens to be Halloween and it happens the invaders are only about 4 feet tall. As the bumbling aliens wander around the countryside they are taken to be children and they make friends with two children, one of whom is the daughter of the sheriff. As their troubles mount (it's difficult for five aliens to conquer a world) they begin to give up their plans of conquest, but then there is that nasty killer robot.

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