Dr. David Fielding and his assistant, Dr. Web Spencer, succeed in landing a communications device on Mars but are puzzled by the tremendous strength of the radio signals emitted just after the device mysteriously explodes. David leaves Cape Canaveral, Florida, to vacation on a friend's estate in Los Angeles, California, with his wife, Claire, who is contemplating leaving him because of his negligence, and their two children, Judi and Rocky. Upon his arrival David discovers his own double and doubles of the members of his family roaming the estate. He learns that the lookalikes are Martians whose mission is to prevent further landings on Mars by reducing to ashes all those involved in the recent landing and taking their places; and he is unable to prevent the completion of the mission.