After a violent meteor shower, the women and Dr. Smith find the wreck of a spaceship, but whatever was aboard sneaks onto the Jupiter 2 and raids the food supply. Soon, a little dog makes an appearance, but Smith believes it a hostile alien in disguise. Meanwhile, a hairy creature spies the camp and the dog chases after it, but Smith believes the "alien" canine left to summon an invasion force. As Smith arms himself for war, Judy goes out to look for the dog unaware that the monster is tracking her down.