Shy, unassertive researcher Dr. Arthur Zeller (Leland Orser) routinely admires a Homo erectus display in the Life Sciences department of his university and daydreams about what it would be like if he could only unlock the animal within him. It turns out his real life is that of a pushover, a "doormat" for his co-workers. Even a bum shakes him down regularly for "lunch money". All his brilliance and scientific accomplishments take a backseat to his meek personality and no one seems to take him seriously. One of his only solaces is his love of the classical works of a favored composer. He begins to try to make his dreams a reality by developing a kind of gene therapy which involves injecting himself with the genes for dominance extracted from primitive primates. This starts changing his entire personality. Zeller suffers from occasional lapses in which he reverts to the mind of a caveman and attacks his boss (John Novak), and kills and eats a dog.
Unfortunately, the changes may improve his life on many levels but they are doing nothing for his love life. His affection towards his co-worker Dr. Laura White (Jenny Levine) remains unrequited and so he decides to tip the scales. During a routine flu shot session, he surreptitiously injects Laura with the genes for submissiveness. The injection has severe side effects for Laura because her basic personality is not submissive at all, so she starts to pass out. Arthur takes her to his place and attempts to attack her, but regains control of his senses and begs her for help.
He tells her what he has been up to and the shocked Laura decides to help him develop a way of reversing the process. Together they come up with an experimental antidote, but things go really bad when he reveals to Laura that he had injected her with the submissiveness genes.
Seeing that he has blown his chance with her forever, Arthur escapes into the night with a batch of his syringes. Laura has a change of heart and follows him to offer her help, but it's too late for Arthur who had already injected himself with a megadose of the ape genes. She desperately tries to appeal to his humanity with his favorite classical music, but he smashes the tape player; he rejects his thinking, brilliant mind and only wants to feel the senses of his primitive self.
Arthur has now permanently become a caveman in both mind and physical form, finding some food out in the wilderness. However, his primitive mind is unable to comprehend that he is not in fact outdoors, but now living inside the very Homo erectus display he so admired. In her work clothes, Laura watches him in silence on the other side of the glass, then sadly turns and walks away.