This is a typical Paul Naschy movie and as such is difficult to describe to someone who hasn't seen one. On one hand, it is very old fashioned movie hearkening back to classic American thrillers like "Gaslight" or classic European thrillers like "Diabolique". On the other side, it opens with the scene of a completely naked women being chased through the woods by a knight on horseback and then brutally maced (in the medieval sense)into bloody pulp. (People who complain about the sexism and violence toward women in American films ought to take a gander at this).
Naschy has a way of combining disparate horror themes and subgenres in what seems like a very random, everything-but-the-kitchen-sink manner (his werewolf films, for instance, have involved everything from Dr. Jeckyl and Mr. Hyde to Countess Elizabeth Barthory to a Himalayan yeti). In this film he uses the evil Count Marnac he created for an earlier movie, "Horror Rises from the Tomb", but rather than making a conventional sequel he freely re-writes the legend making the knight an avenging figure who punishes unfaithful women (in "Horror" the count's wife was a conspirator in his crimes, here she is his first victim). Then he proceeds to ignore the supernatural elements of the story until the very end to tell a "Gaslight"-like tale of a man (Naschy himself, natch) who is only using the legend to frighten his wife to death. He wants to be with a mistress, of course, but just to needlessly complicate things he has TWO mistresses--the very sexy "bad seed" niece of his housekeeper(insert audible drooling sounds here) and another woman (who seems to only exist to provide additional nudity and and another gory killing, but oh well). The whole thing really shouldn't work, but somehow it does.
The recent Mondo Macabre disc this comes on is one of their best with a very interesting (but lamentably short) documentary that serves as an excellent primer for Spanish horror in general, and a longer, equally interesting interview with the erstwhile Spanish werewolf himself. This is neither the best or the worst Naschy movie I've seen, but this new disc should be a must-have for any Naschy/Spanish horror fan.