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While exploring a cave, students Malcolm, Cliff, and Bill are attacked by an ape-like beast. When Bill is killed, the other two report the incident to the skeptical police and anthropologist Dr. Brockton. The news media learn of the story, and a camera crew is sent into the cave to film the creature, but the beast chases them from the cave. Dr. Brockton, who persuades the police not to open fire, shoots the beast with a tranquilizer gun and takes it to her laboratory where she discovers that it is a troglodyte, the missing link between man and ape. After Trog is calmed, a transmitter is inserted in its chest, and the beast begins to communicate with Dr. Brockton. One day a dog steals one of Trog's toys, and the creature kills the dog. Consequently, Dr. Brockton is brought before a magistrate and told that the beast will have to be destroyed if it breaks loose again. Sam Murdock, a local land developer, and Dr. Selbourne, a jealous rival of Dr. Brockton, conspire to free Trog, but when Murdock opens the cage, Trog kills him, goes on a rampage in the town, and takes a child back to the cave. The British Army is called in, but Dr. Brockton requests an opportunity to try to save the child; she enters the cave and returns with the child, who had been treated kindly by Trog. The army, however, will no longer allow the potential danger, and they destroy the cave and Trog with dynamite.