Three spelunkers explore a cave in a recently opened fissure and encounter a troglodyte (trog, for short) - an Ice Age, missing link, half-caveman, half-ape. Trog kills one caver and injures another. The injured party is taken by the third member to Dr. Brockton's nearby research center for care, where the prominent anthropologist is immediately interested. Because of the death, the police came to investigate. During this procedure, Trog follows a frogman through a watery passage to the cave exit. Brocton uses tranquilizer darts to subdue him and bring him to her facility. Once there, she and her daughter Anne begin to domesticate him. He learns to play with dolls, is introduced to the names of colors and dances to soothing music. A colleague performs thoracic surgery on Trog so that he can even start to speak a little. Sam Murdock, a local real estate mogul, whose plans for a housing development are put on hold by the "monster", breaks into the lab at night where Trog is kept. He trashes the lab, hoping the blame will fall on Trog. He then releases Trog, hoping that the beast will go on an ensuing rampage. Trog is so upset by Murdock that he ambushes him at his car and kills hem before starting into the village. In town he wrecks a produce stand, hangs a butcher on one of his meat hooks and snatches a little girl from the playground. Trog takes the girl back to his cave where he hides until authorities arrive to take the situation in hand.
—Garon Smith