The summary of the 1950’s Hammer movie based on it (which tells you all you need to know) tells the story:
”When the Duc de Richleau (Christopher Lee) and Rex Van Ryn (Leon Greene) arrive at a fashionable party thrown by de Richleau's protégé, Simon Aron (Patrick Mower), they soon realize that the party is in fact a gathering of a Satanic cult, led by the high priest Mocata (Charles Gray), that plans to initiate the beautiful Tanith (Nike Arrighi) that night. It's up to de Richleau and Van Ryn to defeat the devil-worshiping Mocata and save innocent young Tanith and the others from a terrible fate.”
That’s pretty much it. The best part of the book was the 1930’s environment it was set in, complete with cars, manor houses and airplanes. It wasn’t bad at all – it was about like watching an old episode of MacGyver. Or, candidly, a dramatic version of a Scooby Doo episode. That’s just true horror fiction though. It wasn’t remotely scary or supernatural.