This was Naschy's 2nd film working with director Leon Klimovsky, following their hugely successful 1970 collaboration La Noche de Walpurgis.

This film also featured Euro-Horror star Jack Taylor, Mirta Miller and the beautiful Shirley Corrigan of England.

A young, wealthy Spanish landowner, Waldemar Daninsky, aka "El Hombre Lobo" (The Wolfman), searches for a cure to his lycanthropy.

Unfortunately, the procedure results in an even more savage monster than before, since the werewolf only killed against his will, but Mr. Hyde actually enjoys the sadistic acts he commits.

Henry Jekyll winds up getting stabbed to death by Sandra, a jilted lab assistant/ lover, and the Wolf Man is shot dead with silver bullets, fired by Justine, a woman who loved him enough to end his torment.