Luisa (Carmen Yazalde, who is billed as Britt Nichols, and also shows up in Franco’s Jungfrauen-Report, A Virgin Among the Living Dead and Dracula, Prisoner of Frankenstein) learns from her dying mother that her family — the Karlsteins not the Karnsteins — are all vampires and their leader, Count Karlstein still lies in half-dead, half-alive stasis in the crypt of their castle.
This being a Jess Franco movie, Luisa is soon taking her cousin Karine (Anne Libert, Sins of the Flesh) as a lover with just as much gusto — in front of a holy cross — as she throws victims to the Count (Howard Vernon), who stays in his coffin all of the time.
Franco also shows up as vampire killer Cyril Jefferson, who takes over the movie as the two main characters suddenly are no longer the leads. It also becomes a giallo, so a movie that feels like two films at once was ironically shot at the same time that Franco was making two other movies.
You can watch this on KinoCult.