Awakened during a thunderstorm, youngster David MacLean witnesses a brightly lit flying saucer disappear underground in the large sand pit behind his home. When his father investigates, he returns a changed man; soon David's mother, a young neighbor girl, and others begin to act in the same way. Begging the police for help, David's panicked story is heard by Dr. Pat Blake, who takes him to astronomer Dr. Stuart Kelston. David soon convinces Kelston, who comes to believe this is an invading vanguard from Mars.