Santo vs. Frankenstein’s Daughter aka Santo contra la hija de Frankenstein opens Tuerto (translated as “One-Eyed”) and some other men exhume a body to be used by Dr. Freda Frankestein and her assistant, Dr. Yanco.
Frankenstein is constructing a creature out of different corpses, and hopes to bring it to life. Another of her creations, Truxon, is a half-man, half-“beast” (gorilla), who eats raw meat and is kept under control by Freda’s hypnotic powers. He will soon regress entirely to an animalistic state.
Frankenstein has problems of her own: she, Yanco, and their assistants are all very old, but retain youthful appearances and vigor due to a serum that the mad doctor has invented. However, not only is this serum painful to take, it has to be administered ever more frequently.
Freda tells Yanco that Santo is the solution. She saw him perform in the ring 30 years ago, and then again recently: he was as strong and agile as ever. She believes Santo’s blood contains a super-concentration of a certain factor that prevents aging. She acquires some Santo blood from a handkerchief he uses after a match to clean up a nose bleed. With more blood from the silver-masked man, Freda and her assistants can perfect their youth serum.
Meanwhile, Santo is wrestling the red-masked El Toro, from Argentina, in a world middle-weight championship semi-final. At home, Santo’s girlfriend Norma is watching the match on TV. Her sister Elsa asks how Norma could fall in love with a masked man. He takes off his mask when we are alone, Nora replies, and “if you could see him without his mask, you’d be as crazy about him as I am.” Despite El Toro’s blatantly illegal use of brass knuckles Santo wins and will now wrestle for the title.
In order to lure Santo to her lab, Freda sends her men to kidnap Norma. They leave a note telling Santo what to do. He and Elsa, who insists upon coming along, travel to the little town of Santa Fe. The sheriff is don Elías, who tries to discourage them from going further into the wilds at night, but they press onward. As Santo and Elsa trudge through the woods towards Freda’s headquarters, they hear a howl, which Santo says is a hungry coyote. But he tells Elsa not to fear, “coyotes only attack deer, sometimes men, but never pretty girls.”
Meanwhile, Freda has succeeded in bringing Ursus, her new monster, to life. His face and boxy head somewhat resemble the traditional Frankenstein’s monster. Freda also hires six new employees. They are all elderly men, who will get access to the youth serum if they serve her faithfully. She warns them that the injections will be painful, “as if your blood is boiling,” and we hear screams as the treatments begin. Also on the youth serum payroll is don Elías, the treacherous sheriff, who doesn’t look old now, but needs his fix for later.
Freda also punishes one of her henchmen, Escorpio, for his carelessness which could have betrayed their operation. Despite his pleas, she allows him to get old, older, oldest, right in front of his pals. After he dies and his corpse turns into a sort of mummy, he’s propped up in a coffin stored in an underground passageway.
Ursus breaks free of his restraining chains and roams through Freda’s dungeons. He smashes down the door to Norma’s cell, but Tuerto seizes a torch and chases the monster away. Norma takes advantage of the confusion to flee. She runs into the forest just as Santo and Elsa are approaching the house which hides the entrance to Freda’s headquarters. Santo fights some of Freda’s men; Elsa is caught by Freda (who temporarily turns old, then suddenly is young again). After another struggle, Elsa runs off but Norma and Santo are taken prisoner.
Elsa, unfortunately, decides that don Elías can help her. She arrives during one of his old spells, and is shocked by his aged features. She winds up in a jail cell, and Elías spills the beans about Freda and her plans.
Meanwhile, Freda is taunting a chained-up Santo. She asks him if he is ashamed to be the prisoner of a fragile woman, and he replies “I don’t think you’re fragile–or a woman.” Freda replies, “Would you like to find out? We could be friends…” She takes off Santo’s mask and kisses him, then slaps his face when he rejects her advances. She hands him the key to his chains, and says he’ll fight Truxon.
Man-gorilla Truxon puts up a good fight, but Santo prevails, whipping his opponent savagely with some chains. Freda has her losing monster shot and orders Santo to be chained up again. She then hypnotizes Norma. Giving the young woman a knife and a pan, Freda instructs Norma to cut out Santo’s eyes and bring them to her! In a nice scene, a helpless Santo tries to convince Norma not to de-eye him, she lunges at him, there is a cut to Tuerto waiting outside the cell, and Santo’s agonized scream is heard.
However, Santo managed to snap Norma out of her trance, and he frees himself as well. A fight breaks out in Freda’s lab between Santo and Ursus, with Norma and the others contributing to the confusion. After whacking Ursus with a metal chair, Santo grabs Norma and they run out, taking the secret tunnel Norma used earlier.
While resting in a nearby cemetary, Santo and Norma are surprised by Ursus and another man, who come out of a hidden entrance in a crypt. Ursus winds up impaled on a sharp grave marker. Elsa has escaped from jail, and she meets up with Santo and Norma in town. Their car is out of commission, so Santo gets them a ride to the city with a passing Santo fan in a Jeep. Santo himself goes back to the lab to prevent Freda from fleeing.
Before he gets there, Santo runs into the wounded Ursus. He uses his shirt to bandage the creature’s horrible chest wound, and then enters the crypt that leads to Freda’s lab. However, he runs into a trap in the corridor and collapses. Ursus has followed, and comes to Santo’s aid.
Meanwhile, Norma and Elsa have been recaptured by Freda’s men. Freda threatens to throw acid on the young women’s faces. Santo bursts in, followed by Ursus. Both Freda and Santo try to control the monster, ordering him to kill the other person. Ursus chooses to work for Santo, and strangles Freda. The automatic lab-destruct switch is thrown, and Santo runs out with Norma and Elsa. Dr. Yanco kisses Freda’s dead as the lab explodes.
The film concludes with Santo wrestling for the championship, as Norma and Elsa cheer him on. He defeats Japanese grappler Yamaguchi and holds up the championship belt.