I have memories from my youth of Dennis Wheatley collections, advertised in the back of the Sunday supplements. I seem to recall a photo of a nekkid blonde chick, with her hands tied behind her back, used to promote these. When I eventually got to read the stories at college, I was rather disappointed - and not just since there was no nekkid blonde chick in them, also because they really weren't very well-written, with a reliance on coincidence and contrivance that rendered them implausible. The same is largely true here. There is a great performance at its core from Lee, as the "white occultist" Duc de Richleau [I can see why Lee often states this is among his favourite roles], large chunks of this are difficult to believe, even allowing for the suspension of disbelief necessary to begin with, for any film about black-magic.
The Duc and his friend Rex (Greene) go to pay a visit to their friend Simon (70's TV icon Mower), who seems to have dropped out of circulation, and find him involved with an "astronomical society" run by Mocata (Gray). You can't pull the wool over the Duc's eyes, however. He soon spots it's actually a satanic cult [What gave it away? Was it the massive goat's head mural on the floor? Or the sacrifical animals locked in a hamper?] and manages to convince the sceptical Rex of this, after an encounter with a djinn. Our pair set out to rescue Simon, and co-n00b Tanith from Morcata's clutches, with the assistance of the Duc's neice and her husband (the latter being Paul Eddington, also recognizable from The Good Life). Needless to say, Mocata isn't standing for that, and fights back with all the powers at his disposal, both his own and those of the creatures he commands, up to and including the Angel of Death himself.
The dark arts are terribly genteel here. Up until the final confrontation (which might not actually take place, in a monstrous cop-out), it simply feels like Mocata is exercising freedom of worship: the "Satanic ritual" from which Simon and Tanith are rescued looks more like a chill-out rave than anything. Morcata pops round for a friendly chat with the niece and to exercise his hypnotic powers, but hardly seems dangerous, except when taking his leave: "I shall not be back. But something will..." That's a rare moment of menace in a film that is otherwise almost entirely non-threatening, and largely flops flat as a result, despite the best efforts of Lee, who gets a rare chance to play the hero in a Hammer movie, and makes the most of it. Shame it was in one of the studio's least-impressive scripts.