Much like the Re-Animator (who’s popularity I imagine this film tried to play off of), Rejuvenatrix covers the issue of messing with a part of life that should not be meddled with. In the Re-Animator, it was death, but in this movie the concept of eternal youth and beauty is the main focus.
The fountain of youth has long been a holy grail, either as a literal fountain out in a remote location, or has a solution that can be discovered. As with The Granny, it never ends well and usually a zombie/demon/monster is the result which will stop at nothing to kill others.
In this story, a rich older woman is privately funding research into an anti-aging formula. Her team of two scientists are dissecting brains in an attempt to fulfill this request. Much money has been invested, with little to show for it. As she grows impatient, she jumps at the first chance to test the product, even though it isn’t fully tested, and only gave initial signs of success in one lab rat.
With much reluctance, the scientist gives her the formula and a few days later after the transformation is complete, she is over 20 years younger and ready to hit the night life and live. After a period of success, she suddenly starts to revert. Not to her old self, but to a horribly mutilated monster. The initial stages are rather gross, what with the lack of hair and pulsating cheeks, but after the doctor gives her a quick fix via another injection, each return to the monstrous form takes on new levels of horror.
As each injections proves to be less effective, and the side effects more intense, the scientist discovers something new. Human brains work much better, and the animal brains he tested earlier seem to be more effective as well. Combining this logic would dictate that fresh human brains would produce a much better solution. However these are not the easiest to obtain, especially at the frequency needed.
Naturally a lack of moral fiber and a strong desire to be beautiful end in a murderous rampage as the women finds that she doesn’t even need to refine the brains into a solution, she can just eat them as-is. This goes beyond the original intent, and the scientist wants out, be she’s not about to let him get in her way.
While not a complete copy of the Re-Animator, you can tell they wanted to ride the coat tails a bit. But it does deliver on the visuals, and the prosthetics are rather interesting. There is a twist at the end, and naturally it isn’t a happy one.