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On the Dunsfield University campus, student Jimmy Flanders delivers the corpse of a coelacanth fish from Madagascar to paleontology professor Dr. Donald Blake. After stopping his dog, Samson, from licking the water running from the fish's tank, Jimmy helps Donald carry the fish into the lab, where the professor explains that the coelacanth species began over one million years ago, and has somehow resisted evolution. Moments later, Samson, a normally friendly German shepherd, viciously attacks Donald's fiancée, Madeline Howard, forcing Donald to cage him in the laboratory. That night, Molly Riordan, the assistant to veterinarian Dr. Oliver Cole, stops by the lab to pick up a sample of Samson's saliva so it can be tested for rabies. Ignoring Molly's flirtatiousness, Donald shows her that Samson has developed long, wolf-like fangs, and then asks her to help him move the fish. In the process, Donald cuts his hand on the fish's teeth, then accidentally dunks the wound into the tank water. He immediately feels ill, so Molly drives him to his house, where he passes out. As Molly calls Dr. Cole, a monster attacks her. Soon after, Madeline drives to Donald's house, and finds it ransacked, with Donald lying unconscious on the lawn and Molly's dead body beside him. Police lieutenant Mike Stevens heads the investigation, and although he at first suspects Donald, who has no memory of the evening, he finds huge handprints which clearly indicate another being was in the house. Days later, Donald teaches his students that man's pursuit of knowledge can either reduce the race to bestiality or increase humanity's understanding beyond our imagination. After class, Jimmy and his girl friend, Sylvia Lockwood, check on Samson, who is now placid. When Dr. Cole comes to the lab, Donald tries to point out Samson's fangs, only to discover that they have disappeared. Distracted by Madeline's concern about his relationship with Molly, Donald reassures her that they were just friends. Later, Mike, who believes that someone is trying to pin the murder on Donald, posts Sgt. Eddie Daniels to guard him, and informs Donald that Molly died not of wounds but of fright. Donald continues his experiments, and while Eddie shoos off a dragonfly that is biting the fish, Donald ascertains that the fish's bacteria are crystallizing, a scientific impossibility. He rushes to Dr. Cole's, but the bacteria suddenly return to normal, prompting Dr. Cole to wonder if the attack has caused Donald to hallucinate. Donald returns to the lab, and when Jimmy and Sylvia stop by, the trio see a gigantic flying insect, which Donald identifies as the prehistoric ancestor to the dragonfly. Jimmy helps Donald kill it, then leaves with Samson, promising not to tell anyone about the dragonfly until Donald can study it. Unknown to Donald, while he is transporting the huge bug, some of its blood drips into his pipe, and when he smokes it later, he transforms into the monster again. Consumed by base instincts, Donald kills Eddie when the sergeant returns to the lab, leaving huge footprints behind. Hours later, a confused Donald theorizes to Mike, Madeline and her father, college dean Gilbert Howard, that a "sub-human" being has been terrorizing the campus. They disbelieve him, however, spurring Donald to spend the next weeks frenetically trying to prove its existence. He cancels his classes and places long, expensive phone calls to an expert in Madagascar, from whom he learns that the fish was preserved in radioactive gamma rays. Donald realizes that this proves his theory: that the fish has resisted evolution through its plasma, which, once radiated and transferred to another organism, causes the second organism to revert to an earlier stage of evolution. While pondering in silence how someone would have become infected by the fish plasma, he suddenly realizes that he himself has been contaminated twice, and in horror, accepts Gilbert's recommendation that he take a leave of absence. He goes to Gilbert's mountain cabin, where he secretly records his suspicions that he is the beast, and his intent to photograph himself in process of transformation. Meanwhile, Jimmy and Sylvia learn that Gilbert considers Donald unbalanced, and so confess to Madeline that they witnessed the dragonfly's transformation. Alarmed, she contacts her father and then drives to the cabin, at the same time that Donald injects himself with plasma. By the time Madeline drives up, Donald, as the beast, leaps in front of her car, causing her to crash. He is about to attack, but upon recognizing her, relents and carries her into the hills. Ranger Tom Edwards sees them, however, and after calling Mike with a description of a "half-man," draws his gun and chases Donald. Madeline awakens and screams, alerting Tom to their location, but when he tries to save her, Donald kills the ranger with the blow of an axe. Donald then lapses into unconsciousness, and later awakens to find Madeline in the cabin, unharmed, with Dr. Cole, Gilbert and Mike in attendance. Donald develops the photo in private and, when it confirms his worst fears, he tells Madeline he loves her, then leads the three men into the hills. There, he pulls Dr. Cole aside and admonishes him that humanity has not yet left the past behind, and therefore must discover and destroy the savage in modern man, then injects himself. Although Dr. Cole calls to Mike not to shoot, the policeman kills the beast, as Donald has planned. The three men then watch in horror and grief as the dead creature's face metamorphoses back into Donald's.