Science professor Raymond Bond is monitoring the orbit of the mysterious "Planet X," which has entered the solar system and is traveling close to Earth. A spaceship from this planet lands on earth, and an alien has come to make contact with friendly Earthlings. However, evil scientist William Schallert wants to exploit the spaceman's highly developed intellect for his own selfish ends. Schallert's nastiness turns the alien against the other Earthlings; the creature enslaves their minds and transforms them into zombies. Both Schallert and the alien are eventually destroyed—as Planet X, failing to establish a bond with Earth, spirals off into deep space.