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Siblings Johnny and Carla are playing outside and pretending to be spacemen when they encounter archaeologists George and Roy at the entrance to a cave. The men explain to the children that they are examining ancient pictographs. Moments later, the children's mother Martha, a widow, and their adult sister Alice are relieved find them safe and take them back to their picnic site. Later, Johnny awakens from a nap and finds that he is alone: When he runs back to the cave he is knocked out by a lightning blast, after which prehistoric dinosaurs appear and begin to fight. After the dinosaurs disappear, Johnny revives and discovers strange machines by the cave entrance, one of which emits bubbles. Moments later, a creature appearing to be half-beast and half-robot emerges from the cave. The beast, actually an alien named Ro-Man XJ2, uses a viewing screen to report to the Guidance Ro-Man on another planet that he has eradicated humankind, as they constituted their only planetary rivals. Ro-Man reveals that although humans had developed atomic energy, his cosmic rays were more powerful. However, Guidance tells him that their computers have determined that there are some survivors, and he orders Ro-Man to destroy them. When Johnny runs home, George, who is now his father, reprimands the boy for leaving the electrified compound, which he and Alice constructed so that the Ro-Man cannot detect them. Moments after Johnny reveals that Ro-Man is nearby, the alien contacts them on a viewing screen and promises they will die painlessly if they surrender. The family defies him and he again contacts Guidance, who threatens punishment if he does not destroy the eight surviving humans. Roy then arrives at the compound on foot and reveals that there are two additional survivors making their way to a rocketship. Roy believes that an antibiotic serum, which George had tested on him and his family, has acted as a vaccine against Ro-Man's lethal rays. Despite an antagonistic relationship, Alice, an engineer, and Roy work together to re-wire their viewing screen so they can contact a space station as yet untouched by Ro-Man. However, they are unsuccessful and Ro-Man destroys the space station and the rocket carrying the other two survivors. During their second meeting via the viewing screen, Ro-Man takes an interest in Alice and demands that she meet with him. Rather than allow Alice to sacrifice herself, Roy restrains her, but later releases her to look for Johnny, who has run away to confront Ro-Man himself. Ro-Man explains to Johnny that humans were destroyed in order to prevent them from becoming strong enough to attack the Ro-Man homeworld. When Ro-Man attempts to kill Johnny, the boy innocently reveals that his family is resistant because of George's serum. Ro-Man now plans to adjust his machinery to counteract the serum, and Johnny, realizing his mistake, runs home and admits he gave away their secret. Alice and Roy, meanwhile, hide from Ro-Man in some shrubbery where their antagonism turns to love. When they return home, George marries them in an impromptu wedding ceremony. When Alice and Roy walk into the hills for a brief honeymoon, Carla follows to give them flowers, and is captured and killed by Ro-Man. Ro-Man contacts Guidance with his news and asks that they allow one human to survive as a sample, but his request is denied. Ro-Man soon encounters Roy and Alice and although they resist him, Ro-Man seizes Alice. Roy survives long enough to warn the family, who are visiting Carla's grave, then dies. Ro-Man, meanwhile, attempts to seduce Alice but is interrupted when George, Martha and Johnny contact him on the view screen and surrender. Guidance reprimands Ro-Man for violating his orders to kill Alice, and for thinking as an individual, which is contradictory to the Ro-Man way. Guidance then orders Ro-Man to kill the family or be killed himself. Johnny, meanwhile, is given an atomic gun by his parents and, in accordance with a plan they have conceived, prepares to sacrifice himself to save Alice. When Johnny appears at the cave, Ro-Man tells Guidance he will kill the boy, but is reluctant to kill Alice. Although Ro-Man follows orders and kills Johnny, he is killed by Guidance. Guidance then directs powerful rays at the earth that will create prehistoric reptiles capable of destroying any remaining human life, and finally, destroy the the earth itself. In the present, Johnny, who has been knocked out by an accidental bump on his head, awakens and is relieved to find that everyone is well and that the preceding events were all a dream. Martha invites George and Roy to dinner while Alice assures Johnny that there are no such things as spacemen. After they leave, however, a Ro-Man emerges from the cave.