Biochemist Dan Scott, working under the premise that all living organisms can heal themselves by adapting to harmful things in their environment, is trying to develop a new serum based on the fruit fly, the most adaptive of all organisms. Dan has successfully administered his compound to dogs, cats and chimpanzees, but oddly, when he injects it into a leopard, the animal's fur changes color. When Dan tries to enlist his mentor, Dr. Richard Bach, in finding a human subject for his experiment, Bach warns that the outcome will be unnatural and possibly dangerous. Nevertheless, Bach offers to help find a dying patient who will consent to be Dan's subject. During rounds at the Grand Mercy Hospital one day, Bach comes across Kyra Zelas, a woman dying of tuberculosis, and refers her to Dan. Kyra willingly agrees to be injected with the serum, and six hours later, she is almost totally cured. On the day that Kyra is to be released from the hospital, Bach invites her to stay at the house he shares with Dan and his housekeeper, Hannah. After fiercely declaring that she will do exactly as she pleases, Kyra accepts Bach's offer. Both Bach and Dan are stunned at the personality change in their formerly timid patient. On her way to the Bach home, Kyra is drawn to a window display of an exclusive dress shop and ventures inside. After an elderly man pulls out a handful of money to pay for his girl friend's dress, Kyra smashes him in the head with an ashtray, snatches the cash and runs into one of the dressing rooms. When the police arrive to search the store, Kyra dons a dress hanging from a hook and wills her hair color to change from black to blonde, causing the store owner to fail to recognize her as the thief. That night, when Dan and Bach return home, they are greeted by Kyra, now a sultry blonde. Claiming that she decided to bleach her hair, Kyra offers to show them her new wardrobe. Although Dan is smitten by the new Kyra, Bach is suspicious and steals a strand of hair from her brush. Upon examining the hair under a microscope, Bach discovers that the color is natural and not bleached. Bach's concerns are confirmed the next morning when he sees a newspaper story about the dress store theft, containing a picture of Kyra's discarded dress, which is identified as the garment worn by the thief. Recognizing the dress, Bach questions Kyra about the theft. After she insists that she is innocent, Bach confronts her with the fact that she lied about bleaching her hair. Admitting to the crime, Kyra dares Bach to stop her. After overhearing Bach and Dan discussing the possibility of developing an antidote to the serum, Kyra enters the lab, seduces Dan and smashes his test tubes. Later, at a party hosted by Bach for hospital board members, Kyra sweeps down the steps in a low-cut dress, enticing wealthy playboy Barton Kendall. Kendall and Kyra sneak out of the party into the garden, where Kendall's wife Evelyn catches them in an embrace. Evelyn calls Kyra a trollop and then slaps her. As Kyra eavesdrops from the terrace, Evelyn and Kendall argue and Kendall asks for a divorce. After Evelyn refuses and Kendall returns to the party, Kyra wills her hair to turn black, slips on a cape and strangles Evelyn to death. When Bach finds strands of black hair clutched in the dead woman's hand, he accuses Kyra of murder. Readily confessing, Kyra warns that she will kill anyone who stands in her way. Concluding that the serum may have over-stimulated Kyra's pineal gland, thus turning her into a "devil," Bach proposes that surgery may render her normal once more. To ensure her cooperation, Bach and Dan try to drug Kyra in her sleep, but she awakens and threatens to kill them both. The next morning, Kyra is gone and Dan, in love with her, blames Bach for driving her away. Kyra continues her pursuit of Kendall, and one day, Bach and Dan read about their engagement in the paper. When Dan visits Kendall to warn him about Kyra, Kendall accuses him of acting out of jealousy. Kendall and Kyra are wed and upon returning from their honeymoon, Kendall cloisters his wife in his hunting cabin. Growing intolerant of her husband's sexual advances, Kyra begins to spurn him. When Kendall threatens to change his will and forces himself on Kyra, she pushes him away and he grabs his gun and shoots her. Although her wound instantly heals, Kyra hides her recovery from Kendall, and he insists on driving her to see a doctor. Along the road, Kyra seizes the car's steering wheel and drives it over a cliff, emerging unscathed from the wreckage. Upon learning that Kendall has been killed, Bach and Dan resolve to stop Kyra. Feigning sympathy for the widow, they invite Kyra to spend the night at the Bach house. After Bach deduces that carbon dioxide will anesthetize Kyra, they pump gas into her room through the radiator, and once she falls unconscious, they operate on her pineal gland. Kyra awakens from the anesthetic, her hair raven black, on the verge of death from advanced tuberculosis.