Santo vs La Hija De Frankenstein (1971)

AKA: Santo vs Frankenstein's Daughter.

Dr. Freida Frankenstein.

Frankenstein's daughter is alive and well thanks to her youth serum. Unfortunately for her, she's developing a resistance to it and she needs Santo's blood to help her develop a better serum before it fails and she dies of old age. Rather than just asking Santo for a sample of blood, she kidnaps his girlfriend to lure him to her underground lair where he must battle Dr Frankenstein's once again re-animated monster and a small army of youth-serum dependent elderly henchmen.

Santo and his girlfriend's sister.

I absolutely love these old lucha-libre films, but this one is perhaps the most fun of the Santo films that I've seen to date (I'd place Santo en la Venganza de la Momia a close second).

Besides the near constant fights between Santo and Dr. Frankenstein's henchmen, this movie is loaded with bizarre fun stuff- the entrance to Dr Frankenstein's lair is lined with mummified corpses- corpses that shoot knock out gas from their eyes! And there's a great big self destruct switch on the laboratory's wall. Seriously, this movie is campy, comic book style fun.

Imdb