Irena Gallier, portrayed by Nastassja Kinski, is a young woman raised by adoptive parents who reunites with her estranged older brother, Paul, played by Malcolm McDowell, in the vibrant city of New Orleans. Their first encounter unfolds at the airport, where Paul warmly welcomes Irena and drives her to his apartment, which he shares only with his housekeeper, Female, pronounced Fee-MAL-ee (Ruby Dee). Unbeknownst to Irena, both she and Paul are part of a lineage of 'cat people,' individuals who transform into panthers after engaging in sexual relations and must kill to regain their human form.
Irena, innocent and a virgin, is oblivious to her unique inheritance. In stark contrast, Paul has come to terms with this dark secret and has developed a method of luring women as prey. On Irena's initial evening in New Orleans, Paul makes plans to meet a sex worker (Lynn Lowry) at a seedy hotel. However, the encounter takes an unexpected turn when the woman escapes, leaving Paul stuck in his panther form, confined within the hotel walls. As chaos ensues, Curator Oliver Yates, played by John Heard, along with his team members Alice Perrin (Annette O'Toole) and Joe Creigh (Ed Begley Jr.), is summoned to capture the rogue panther.
With Paul absent, Irena spends her first full day exploring New Orleans. She enjoys a taxi ride through the charming French Quarter and visits the zoo, where she becomes entranced by a black panther, losing track of time as she sketches it long after the zoo closes. Startled by Oliver, who later introduces himself, Irena drops her sketchpad and scurries into the trees but is coaxed down by him. Upon noticing her feverish state, Oliver applies a cool cloth to her forehead, and they decide to grab a bite together, during which Oliver learns of Irena's job search and offers her a position at the zoo's gift shop.
Irena's inaugural day as a zoo employee is both eventful and alarming. As she connects with Alice over lunch, a mysterious woman approaches and refers to Irena as 'mi hermana' (my sister). Later, during her shift, the black panther behaves aggressively, leading to a horrifying incident where Joe loses his arm and dies from severe injuries, leaving the panther nowhere to be found when Oliver returns with his firearm.
That night at home, Paul reappears, his demeanor less than brotherly. He claims he was "in jail" during his absence and attempts to seduce Irena, insisting that their unique biology demands they share intimacy. However, Irena finds herself drawn to Oliver and rejects Paul's advances, fleeing in fear. She manages to attract police attention, leading them to Paul's basement, where a shocking discovery of chains and human remains points to his disturbing past, marked by mental illness and familial tragedy.
As Irena grapples with her troubled feelings over Paul and her growing affection for Oliver, she experiences inexplicable urges, even finding herself sprinting on all fours after a rabbit one night. Meanwhile, Paul continues his predatory behavior, culminating in yet another murder. In a desperate confrontation, Paul crashes into Irena's room, once again pushing for intimacy, claiming that their path mirrors that of their deceased parents. Defending herself, Irena stabs Paul with broken glass, triggering his transformation into a panther. Oliver and Alice intervene, and in an act of self-defense, Alice kills the panther, only for Oliver to discover the horrifying truth of its human origin during the autopsy.
The next morning, while Alice jogs and stops for a swim, she feels a sense of foreboding when she hears growls. Stuck in a panic, she jumps into the pool, but her fear escalates when Irena appears, with her disheveled clothes raising alarm. After charming Oliver and revealing her readiness to consummate their relationship, Irena transforms into her panther alter ego yet again, leaving before causing harm. While SWAT teams pursue her, Oliver searches for Irena, only to find her in human form, pleading for release through one final encounter.
In a haunting conclusion, the story culminates as Oliver visits the zoo, confronting the trapped panther that is now Irena, forever confined within her bestial form. He tenderly reaches through the bars to caress the docile creature's neck, symbolizing a tragic love forever shrouded in darkness and despair.
