There is a Satanic coven holding a human sacrifice one night in the wilds of the English countryside, and their leader, Alexander Yorke (Michael Gough) enthusiastically plunges his dagger into the naked body of... a young woman who he may well know all too well. Elsewhere, sometime later his son Stephen (Martin Potter) is romancing a new woman in his life over a glass of wine at the family mansion in the woods. The evening goes well until he takes her up to his room, when suddenly he turns into what she describes as "an animal", and she flees - but not fast enough, as he catches up with her...
The surprisingly short filmography of British auteur Norman J. Warren got its first truly identifiable horror "hit" in the shape of Satan's Slave, written by sometime critic David McGillivray in the tawdry manner of his scripts for Pete Walker. For a shocker out of the United Kingdom at the time, it was surprisingly bloodthirsty, something closer to the continental view of horror as an excuse for naked ladies and gore, but not so extreme that it could not be shown on late night television in its native country well into the twenty-first century.
For some reason, perhaps to add more of a punch to its opening, the film gives the game away about Uncle Alexander's preoccupations with devil-worship from the very beginning, which utterly sabotages any of the twists it so determinedly sets up later on. What appears to be the heroine is killed off in the first ten minutes, so we have to settle anew with Catherine (Candace Glendenning), who is just waking up on the morning of the murder with her boyfriend (Michael Craze). He gives her a bracelet that belonged to his mother as a parting gift, and off she goes to her parents' home to travel into the country and stay with, that's right, Uncle Alexander.
Michael Gough was a past master at this kind of low budget chiller, having appeared in them since the fifties, but he sets about the villain part with a going through the motions approach rather than the relish he was capable of in his more enjoyable roles. I suppose even if there had not been that giveaway introduction, we could have guessed that the old man was up to no good, but as it is we have a lot of waiting around to do before his secretary Frances (or Francis, as the end credits call Barbara Kellerman's character) spills the beans to Catherine about what is really going on in that very big house in the country.
Before it can build to the climax that so many British horror movies led up to in this era, that is someone being chased through the woods, it has to put the heroine through hell, so when they do arrive at the old house, Catherine's father (James Bree) takes a funny turn and drives into a tree. Although they were travelling about five miles an hour, the impact kills her mother and when she gets out of the car to fetch help the vehicle explodes in a highly unlikely plot development. This gives the bad guys all the excuse they need to keep Catherine in their home to recuperate, but she is prone to funny turns herself, or premonitions as she calls them, even though what she is actually seeing are flashbacks. What follows does so at a snail's pace and resembles a Dennis Wheatley movie that the filmmakers had to make up a fresh plot for as they couldn't afford the rights to the real thing. Music by John Scott.