As the Serbian-born fashion illustrator Irena Dubrovna sketches a black panther at the Central Park Zoo in New York City, engineer Oliver Reed strikes up a conversation with her. She invites him back to her apartment for some tea where he notices a statue of a medieval warrior on horseback impaling a large cat with his sword. Intrigued, he asks her about it. She says the figure is King John of Serbia, and the cat is evil personified.
She recounts the age-old legend where the Christian residents of her home village turned to witchcraft and devil worship after the Mameluks enslaved them. King John was able to drive the Mameluks out, but after he saw what the villagers had become, he had them executed. Only the “wisest and the most wicked” among them escaped into the mountains. Even though Irena takes the story seriously, Oliver is very dismissive of it.
Despite that, the pairs’ relationship grows. Oliver buys Irena a kitten, but when the kitty meets Irena, it disses its displeasure at the woman. Irena suggests they go to the pet shop to exchange it. When Irena enters the business, the animals there go wild with fear; making Irena uneasy in the process too.
In time, Irena opens up to Oliver that she thinks she’s a descendant from the “Cat People” of her village which means that she will turn into a vicious panther when her passions arouse her. Oliver, still not believing her, asks her to marry him. She agrees.
During a dinner their wedding night at a Serbian restaurant, a catlike woman walks over to Irena, and says, “my sister” to her in Serbian. Freaked out, Irena refuses to consummate her marriage out of fear she will turn into that killer panther of legend. While Oliver is mostly patient, he still persuades her to see Dr. Louis Judd, a psychiatrist. Judd believes her fears stem from childhood traumas; not from reality.
At the same time, Irena expresses great displeasure over Oliver confiding his marital troubles to his assistant, Alice Moore as Alice is clearly in love with Oliver. When Irena spots the two at a restaurant, she follows Alice home. Alice is spooked when she hears a roaring sound from a large cat. When a bus pulls to her stop, and she quickly boards it.
Nearby, a groundskeeper discovers several freshly killed sheep with large cat tracks leading to the imprints of a woman’s shoes. Irena returns to her apartment confused and exhausted where she jumps into the tub for a good cry. After Irena dreams of Dr. Judd dressed as King John speaking about a key, she steals the key to the panther’s cage at the Central Park Zoo.
Irena, Oliver and Alice visit a museum, and they anger Irena when they ignore her completely. Later that evening, when Alice takes a dip in her apartment’s swimming pool in the basement, an unseen animal stalks her from the shadows. Suddenly after Alice screams for help, Irena appears and turns on the lights. She claims she’s looking for Oliver.
After she leaves, Alice finds her bathrobe torn to shreds from claw-like marks. Irena has another meeting with Dr. Judd, where she tells Oliver she is no longer afraid. Unfortunately for her, it’s too late. Oliver is in love with Alice too, and he intends to divorce Irena for her. Later, as Oliver and Alice work in their office, a snarling animal growls at them from the shadows again. The two manage to get out of the building, but not before Oliver gets a whiff of Irena’s perfume.
Alice calls Dr. Judd to warn him about Irena, but he hangs up when she arrives for her appointment with him. Ignoring her warnings about transforming into a panther, Dr. Judd kisses Irena passionately. She transforms into a panther, and kills the good doctor.
Oliver and Alice arrive, but Irena slips out, and heads to the zoo. There, she opens the panther’s cage with her stolen key. It attacks and kills her before a car hits and kills it outside the zoo. Oliver and Alice walk up to the panther’s empty cage, where they find another dead panther on the ground. It appears that Irena never lied to them.