Cat People(1982)

Short pooper:
Both Paul (Malcolm McDowell) and his sister Irena (Nastassja Kinski) are “werecats”; they have the power to transform, when getting aroused and engaging in sex, into panthers, and revert into human form upon killing another human. Paul himself is behind the killings, having done so in panther form, and is later killed by Alice (Annette O’Toole). Irena’s feline self emerges and, after having sex with Oliver (John Heard), turns into a panther, kills a homeowner, reverts back to human form and, embracing her fate, begs Oliver to help her turn permanently into a panther, after which she is caged at the same zoo where her brother was.

Longer version:
An unexpected sexual advance from Paul disturbs Irena and she runs from him; believing him to be somehow involved in the gruesome killings of zoo caretaker Joe (Ed Begley Jr.) and that prostitute from earlier, she gives thought into turning him in to the Police, but they are alerted nonetheless -due to the rotting smell coming from his basement, caught by a neighbourhood dog- and, upon discovering shackles and human remains, a search is put in place. Meanwhile, Irena looks for Oliver, clearly showing feelings for him but not wanting to give into intimacy.

Paul, now considered a fugitive, shows up to Irena once more and, upon begging her to “make love to him, to stop the killing”, he reveals, not only that HE, HIMSELF, was the killer, but also that he did it WHILE TRANSFORMED INTO A PANTHER and that both he and Irena are “cat people”, whom, thanks to an old ancestral curse, transform into big felines thanks to sexual arousal and intercourse with a human, and can only revert it by killing another human being; moreover, them both as siblings, if they have sex, can stop the power of transformation on each other -their own parents were also siblings-. She flatly refuses, upon which Paul playfully taunts her about “Oliver being in love with the cat, not her own self”; then, after she hurts him with a shard of glass, he transforms once again, tries to attack Oliver and is shot dead by Alice.

As Oliver tries to autopsy the panther corpse of Paul, it spews green gas and a human arm and hand are visible, after which the whole corpse disintegrates into green goo. At the same time, Irena’s latent feline instincts start to manifest, causing her to almost kill Alice. Later, she gives in to having sex with Oliver, thus finally transforming, for the first time, into a panther. However, her humanity and love won’t allow her to hurt Oliver and she flees from him. Police pursue her and, after being cornered on a bridge, Oliver witnesses her jumping off to run for safety. He realizes what she intends to do and catches up with her at a secluded lake house, where he finds her in human form, having killed the house’s owner.

Irena admits to a confused and horrified Oliver that she loves him, reason for which she has not given into her animal instincts and killed him; also, not wanting to live that life anymore, she begs him, first to kill her, and, when he refuses, to have sex with her one last time, so she can transform back into a panther and stay as so forever, “to be with her own kind”. He, then, reluctantly agrees to help her; he ties her, naked, to the bedposts so she won’t escape and has sex with her, which triggers the transformation.

In the end, Oliver is seen working at the zoo and in an unenthusiastic relationship with Alice. He passes by the panther cage, now the home of an Irena permanently in animal form. He gives her some food and gently pets her, to which she -still visibly retaining some degree of human conscience and recognizing him- gives an acknowledging roar.