Trama

A man suffers from the curse of lycanthropy and seeks out the aid of a German doctor and his wife who are experts in the occult. Unknowingly, the cursed man has summoned two vampires instead, who have sinister plans of their own for the werewolf.—Humberto Amador

Rudolph Weissmann [Manuel Manzaneque] and Countess Janice von Aarenberg [Dyanik Zurakowska] are engaged, and their fathers are throwing a masked ball in their honor. They've been friends and lovers since childhood but, when "His Satanic Majesty" Waldemar Daninksy [Paul Naschy] dances with her, Janice falls instantly in love. The next day, when she stops at an antique shop to pick up a vase she ordered, the shopowner formally introduces Janice and Waldemar. Shortly thereafter, Janice runs into Waldemar again when she stops to bless herself with holywater at church. Later, as Rudolph and Janice are driving, they stop at the old, abandoned castle and, lo and behold, Waldemar is there, too, "for the peace and solitude," he says.

Waldemar explains to them the legend of the castle. It was built by the Wolfsteins of Poland. Imre Wolfstein is said to have been bitten by a werewolf and became a werewolf himself. A silver cross was driven through his heart, and he was buried in the tomb in the castle. Legend has it that a werewolf can only be killed by a silver bullet shot into his heart by one who loves him. Therefore, Imre isn't really dead. Janice is intrigued by the story, but Rudolph is peeved at Janice for talking to Daninsky, who has a reputation for being an oddball.

A storm is approaching. On his way back to town, Waldemar stops to help two gypsies, Gyogyo [Gualberto Galban] and Nascha [Rosanna Yanni], push their wagon out of the mud. When they ask if there is someplace nearby where they can take shelter from the coming storm, Waldemar suggests the castle. After having a bit too much wine, the gypsies explore the castle, coming upon the family crypt where lay the tombs of the dead Wolfsteins. Thinking the dead may be buried with jewels, they begin opening the tombs. A ring and a necklace later, they come upon the tomb of Imre Wolfstein. When Nascha pulls the silver cross from Imre's chest, Imre reawakens and tears them both apart.

The next day, two villagers are found dead. It looks like they've been torn up by wolves, so the rest of the villagers form a party to hunt them down. During the hunt, Rudolph is attacked by Imre. When Waldemar jumps in to help, he is bitten by the werewolf just as he plunges the silver cross back into Imre's heart. Out of gratitude for Waldemar having saved his life, Rudolph promises to shelter Waldemar and help find a cure for his condition. Hoping to spare Janice from knowing about him, Waldemar sends her a message to tell her that she must forget him, but Janice isn't put off that easily. She follows Rudolph to the castle where she finds he is keeping Waldemar locked behind bars. Rudolph and Waldemar explain everything to her, and Janice agrees to help them. While rummaging through the library, Janice finds a letter to Imre Wolfstein from Dr Janos Mikhelov dated 1929. In the letter, the doctor intimates that he may have discovered a cure. Rudolph suggests that they contact Mikhelov to see if he can help.

Dr Mikhelov has since passed away, but his son Janos [Julián Ugarte], who has taken up his father's research, arrives at the train station, accompanied by his wife Wandessa [Aurora de Alba]. However, instead of helping Waldemar, Mikelov and his wife, who are vampires, cause Waldemar's body to become possessed by Satan, and they hold him in chains while they feast on Rudolph and Janice, putting them under their power, too. They even manage to pull the silver cross from Imre's heart and bring him back to life, only to keep him chained up next to Waldemar (although Waldemar kills Imre one night when they both manage to break loose of their chains).

Rudolph and Janice's fathers are becoming increasingly worried over the strange behavior their children have lately been showing -- disobedience, wandering about at night, seemingly entranced, etc.. When Rudolph leaves a letter with his father asking for help and Janice's dad finds silver bullets in her bedroom, the dads decide they must take action. They go to the castle, where they hear groaning coming from the cellar. There, they find Waldemar in a cage and release him. When they notice the red pentacle (the mark of the wolfman) on his chest, Waldemar explains how he was bitten by the werewolf and advises them to load Janice's silver bullets into their guns.

Then they all go off to look for Janice, Rudolph, and the vampires. When they come upon the vampires' coffins, Waldemar plunges a stake through Wandessa's heart, thus releasing Rudolph from her bondage. Before they can destroy Janos Mikelov, however, Waldemar begins to change into his werewolf self. Janos escapes with Janice. When Waldemar's change is complete, he catches up with them. Waldemar and Janos struggle as Janice, Rudolph, and the dads watch. Janos is suddenly consumed by fire (that springs up out of nowhere!), releasing Janice from his spell. As Waldemar turns on the dads and Rudolph, Janice grabs the gun from her father and shoots four silver bullets into Waldemar. She then rushes to Waldemar's side as he turns back into his human form and dies. Rudolph leads her away. [Original Synopsis by bj_kuehl.]