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A young boy named Johnny (Gregory Moffett) is on a picnic with his widowed mother (Selena Royle) and sister (Claudia Barrett), when he meets a pair of archeologists (John Mylong, George Nader) exploring a nearby cave. Later, while napping, he has a dream -- that the Earth has been attacked by an alien named Ro-Man (played by George Barrows in a gorilla suit with a diving helmet), using the "calcinator death ray," and that he and his family (with Mylong and his mother now married) and scientist Nader are the only survivors. They try to elude capture by Ro-Man, who turns out to have some very human failings despite his mechanized mentality, including a desire to experience human emotions, which greatly complicates his efforts to destroy the family.