Santo and Blue Demon in The Beasts of Terror aka Santo y Blue Demon en las bestias del terror sees petty crook Pedro and his girlfriend Nora desperately trying to get money to pay back what they owe to a gangster Lucky. They kidnap a girl named Susi, whose sister is a millionaire.
However, they are carjacked by the minions of Dr. Matthews, whose van broke down as they were taking a stolen corpse back to their mad scientist boss. Matthews uses blood transfusions from his captives–including Susi and Nora–to revive the dead bodies. He then sells these submissive zombies to a man wearing a turban.
Matthews is attracted to Nora, and she plays up to him in hopes of arranging her freedom. However, the jealous mad doctor sees her kissing Pedro, and he has the hapless crook thrown into a cell and then has some vicious dogs chew him up. He also has Nora whipped for her duplicity. Susi is molested by Matthews’ servants: first, by the mute moron Rojo, and later by the mute moron Paolo.
Private detective Tony is hired by Lorna, Susi’s sister, to find her. They can’t figure out why the kidnapers don’t call and ask for ransom. Tony promises gangster Lucky a big bonus if he finds out where Susi is being held. Lucky suspects that his former pal Sandro is involved.
Santo and Blue Demon are brought into the case by Tony. After helping Tony escape from the clutches of Sandro and his thugs, Santo says he has to leave for a wrestling match in Mexico. Blue Demon carries on: he identifies Sandro as a former professional wrestler who’s now a crook. Sandro sells Lucky the information about Matthews and Susi, then flees to Puerto Rico, with Blue Demon in pursuit. In a struggle, Sandro is killed, but he managed to rat out his boss before he dies.
Santo comes back to Miami, and accidentally spots more of Matthews’ men robbing a cemetery. He follows them and learns the location of Matthews’ hideout. Santo and Blue Demon decide to attack the house.
Nora convinces Susi to “be nice” to the lovesick Paolo, who is grateful enough to help them escape. However, the two women are caught once more. Matthews is set to pour acid on Nora’s face when Paolo smacks him with a pipe, and is fatally shot for his efforts. Outside, Santo, Blue Demon, Lucky and his gang, and Tony all arrive in succession. While Santo and Blue Demon fight Matthews’ men, Tony shoots all of Lucky’s men. Tony also shoots and kills Dr. Matthews, but one of his stray bullets also kills Nora. Susi is reunited with her sister. Santo and Blue Demon depart.