Late one night, youngster David MacLean (Jimmy Hunt) is awakened by a loud thunderstorm. From his bedroom window, he sees a large flying saucer descend and disappear into the sandpit area behind his home. After rushing to tell his parents, his scientist father (Leif Erickson) goes to investigate David's claim. When his father returns much later in the morning, David notices an unusual red puncture along the hairline on the back of his father's neck; his father is now behaving in a cold and hostile manner. David soon begins to realize that something is very wrong: he notices that certain townsfolk are acting in exactly the same way. Through his telescope, David sees child neighbor Kathy Wilson suddenly disappear underground while walking in the sandpit. David flees to the police station for help and is eventually placed under the protection of health-department physician Dr. Pat Blake (Helena Carter), who slowly begins to believe his crazy story.