After traveling to Africa, a woman learns the secret of eternal youth - unfortunately, it entails regularly sacrificing young men.
Synopsis (From the MST3k wiki)
Dr. Paul Talbot is trying to find the secret of eternal youth. A 152 year-old African woman, Mala, approaches Paul and offers him her tribe's secret to long life, a special orchid pollen, but he is doubtful until she demonstrates how she has kept herself alive. He tells the secret to his estranged, alcoholic, and aged wife, June, and gets an guide to help them go on an expedition to Mala's village.During the dangerous journey, June realizes Paul only wants her along to test the potential potion and becomes enraged. June, Paul, and the guide are captured by Mala's tribe and arrive just in time to see Mala undergo a special ceremony, extracting the pineal hormones of a man with a spiked ring, killing him in the process. She then mixes it with the pollen and drinks it to make her young again. Mala makes an offer for June to use the ring; Paul wants to use the opportunity to escape and "get help". However, June turns the tables and chooses Paul as the man she will kill. She undergoes the ceremony, and she and the guide then create a distraction, steal the ring and pollen, and escape. However, as Mala implied, the effect is temporary; June realizes she must keep killing men to keep young and alive, and so kills the guide after they become lost in the jungle.When she arrives back in the states, Sally, Paul's shrewishly-voiced nurse, is dating Neil, June's wimpish but mildly attractive lawyer. June uses her new youth to pretend to be Terry Hart, her own niece; she constantly hits on Neil and makes Sally utterly jealous. She also goes around town, using her older form, to trick drunks and con men for their sweet, sweet pineal juice. Neil eventually falls in love with Terry, and decides to end it with Sally. Sally confronts Terry and tries to send her to New York via gun; the two fight, and Terry kills her with the ring.During a romantic evening she has planned with the now free Neil, a detective investigating one of the men she's killed comes to her home and confronts her; he discovers Sally's body in the closet. Crazed, June rants about the formula for youth and tries to prove it using Sally's pineal juice, however June does not stop aging - in fact, she begins to age even more rapidly. Realizing she's lost everything, she throws herself off of her second-story bedroom balcony and reveals her horrible secret in death.
Artistic License - Biology: Leaving out the whole "youth serum" thing, the pineal gland is buried deep in the middle of the brain—the small spiked ring used to extract its juice is nowhere near big enough to get at it in a single rabbit punch to the back of the skull.
Asshole Victim: June's last tie to morality after she starts killing seems to be her choice of victims, most of who were crimes of passion. Talbot's an obvious example. Sally was threatening to kill her if she didn't leave. David the hunter is where she first went over the slippery slope, but he did reject her. The one time she kills in completely cold blood, her victim is the con artist who tries to kill her for the jewelry she wore as bait — the film even spends a few minutes showing her rejecting the chance to ensnare the happy drunkard she meets earlier.
But Not Too Black: The actresses who played Old and Young Mala (Estelle Hemsley and Kim Hamilton, respectively). All of the background tribswomen, as well, are noticeably lighter-skinned than the males.
Calling Card: June accidentally leaves one behind after she murders the sleazy guy. Oops.
Crapsack World: The world this movie depicts — every character unilaterally selfish and motivated by their worst impulses — is definitely this.
Economy Cast: There's no story-related reason that Neil needs to be dating Paul's nurse. However, by having him do so, the producers were able to avoid having to cast two separate actresses as the nurse and Neil's girlfriend.
Manipulative Bastard: Paul sees literally everyone around him as nothing but tools for him to use to advance his own ambitions, honeys them up with fake praises and promises only to abandon them the moment they are no longer useful to him. It ultimately leads to his death.
May-December Romance: Sans "romance", in Paul and June's case. June and Neil play it a bit straighter - at least in chronological terms.
Playing Gertrude: June's implied to be approximately in her mid-fifties or so, and much older then Paul. At the time of filming, Coleen Gray was about 37 or 38 - 13 years younger then the man who played her husband. Justified, of course; it's a lot harder to make an older actor look young than to make a younger actor look old.
Too Dumb to Live: Apparently the restoration of her youth shaved off a few dozen of Malla's IQ points. "Well, I had been having psychic dreams about you becoming a murderer, but since I'm young now, I don't really care any more. Think I'll go get laid - seize the day, you know. Anyway, you've got free rein of the village 'til we execute you and your heavily-armed boyfriend tomorrow! Ta!"
Neil Foster probably counts too. A supposedly intelligent lawyer lets himself be instantly seduced away from his fiancee by a client's niece whom he'd never heard of until she appears, and who looks and sounds exactly like a younger version of his client. Not to mention he has every reason to believe she's mentally unstable, given how she behaved earlier.
Yandere: Sally, pulling a gun on your ex's new flame isn't exactly normal.