A young boy named David MacLean awakens during a thunderstorm to witness a flying saucer disappearing underground in the sandpit near his home. Troubled by what he has seen, he struggles to convince his parents of the spacecraft's existence. His father, George, an astronomer, decides to investigate the strange imprint left in the sandpit behind their house. When George returns, David quickly notices a disturbing change in his behavior. His once kind and attentive father has become cold and distant. It becomes apparent that George is not the only townsperson acting out of character; other locals, including the police chief and David's school friends, begin to show similar eerie changes after visiting the site.

Desperate and frightened, David seeks help from Dr. Pat Blake, a physician and family friend. She initially dismisses his fantastic tales but becomes a believer when she witnesses the bizarre conduct of the townsfolk herself. Realizing the gravity of the situation, Dr. Blake contacts her friend, Dr. Stuart Kelston, an astronomer who works at the local observatory. Together with Dr. Kelston, they begin to uncover the shocking truth about a sinister extraterrestrial plot. As more and more people fall prey to the otherworldly influence, the trio scrambles to find a way to expose the truth of the alien incursion before it's too late.