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Late one night in California's Sierra Nevada mountains, anthropologist Professor Clifford Groves investigates a noise in his laboratory and discovers that an animal has escaped through the window. The next day, a hunter named Wheeler is startled by the appearance of what appears to be a prehistoric saber-toothed tiger. Wheeler is later ridiculed when he repeats his story to bar owner Charlie Webb and other local residents, including game warden George Oakes. However, George witnesses the cat firsthand when it leaps across the hood of his car while he is driving through the wilderness that night. The next day, George and sheriff Andy Andrews make plaster casts of the cat's pawprint, which George takes to Los Angeles for consultation with zoologist Dr. Ross Harkness. Ross believes that George is playing a practical joke as the print belongs to the extinct saber-toothed tiger, until George assures him of his sincerity, prompting Ross to join him in the Sierras. Once there, Ross inquires after George at the café, where waitress Nola Mason informs him that George is in the back country, but left his car at the home of Groves and his daughter Jan. As he prepares to drive to Groves's house, Ross agrees to give a ride to Ruth Marshall, Groves's fiancée who is visiting from out of town. Although Groves is in Los Angeles giving a lecture, Jan welcomes the guests and invites Ross to spend the night while waiting for George to return. In Los Angeles, meanwhile, Groves loses his temper and alienates himself from his peers at the Naturalist Society after they dismiss his passionate belief that primitive man's intellect compared favorably with modern man. Intent on quelling their skepticism, Groves returns home and immediately prepares an injection for a domestic cat. When George returns with reports about the saber-toothed tiger, an irritable and protective Groves dismisses the story as ludicrous. George and Ross go into the wilderness and shoot the cat, but when they return to the site with Groves, the animal's body has disappeared. Ruth later privately expresses concern that Groves's obsession with his work is ruining his health, a concern of Jan's as well; however, Groves reacts irrationally and breaks off their engagement. Alone again, he injects himself with a serum that slowly transforms his body into that of a primitive man, then escapes through the window into the wilderness. There, Groves brutally murders rancher Jim Newcomb and his dog. When Groves returns to his house later in the day, his contemporary human form is restored. While Sheriff Andrews forms a posse to find the murderer, Groves documents his transformational process in a journal, also noting his fears that the primitive violent nature is overtaking his rational mind. When he then transforms into the prehistoric man without having had an injection, Groves again leaves the house. While he is away, a suspicious Ross searches the laboratory and develops several negatives that document the progression of a housecat into a saber-toothed cat. Other photos show that Groves injected his deaf-mute housekeeper Celia, who was temporarily transformed into a Neanderthal woman. When Ross asks Celia about the photo, she does not comprehend his questions. The next day, Groves attacks and kills local Buck Hastings while he is on a picnic with Nola, whom Groves then kidnaps. Ross later finds a disturbed Nola outside the house and brings her inside to recuperate. After Ross apprises Jan of the nature of her father's work, he tests his theory by injecting another housecat with some of Groves's serum, but they are interrupted by Dr. Fairchild, who has come to treat Nola. After the doctor leaves, they are alarmed when they hear a disturbance in the laboratory and find the cat missing. After Jan and Ross read Groves's journal and conclude that he is the murderer, they fear that the posse may kill him. Although Groves is sighted by George's posse in a field, they only wound him and he escapes. Believing that Ruth will be Groves's next victim, Ross hurries to Charlie's motel, where she has been staying, and finds that Charlie has been attacked and Ruth is missing. The next morning, Groves retreats into a cave with Ruth, whose pleas prevent the posse from firing at them. Groves escapes but is mauled by the saber-toothed tiger, which is then killed by the posse. At his home, Groves slowly transforms into his normal form as he dies surrounded by his friends and family.