Oliver Reed is the lecherous priest Father Grandier, who sends the local nuns into a tizzy. There's not really a lot of men around and Grandier does have a twinkle in his eye. One of the nuns is the hunchbacked Sister Jeanne (Vanessa Redgrave), who really gets all hot and bothered over Grandier, leading to an awkward prayer ceremony. Jeanne claims that Grandier came to her as an incubus and took her, and all of a sudden the nuns all become demonically possessed.
This posession is less 'evil zombie' and more 'sexual depravity', which makes it much more interesting as a horror film. Derek Jarman's set design of the whole city but the nunnery in particular is of big empty white spaces. What the nunnery instantly calls to mind is a lunatic asylum; even before we really meet Grandier, you see the seeds of madness in there. Redgrave's Quasimodo-like nun is particularly tragic, as she would not be able to cope in the outside world and is forever starved of love and desire through being essentially 'locked up'. Of course she can 'ease' such pangs, though there's always a nun staring voyeuristically through a window.
Although there are some bits which are funny and/or horrific (there was a collective wince when the exorcisers brought out the 'exorcism' tool and when we realised where that would be inserted), it's not simply a bit of seventies camp. There seems to be some intentional innuendo around standard religious sayings- Reed's line ordering a woman to be 'on her knees, before God' sounds shameless- which probably didn't add to its popularity with religious countries. But the link between religion, sexuality and madness feels scarily organic, which is why the film works so well. We are not bombarded with the 'explicit' stuff until the end, so things rightly come as a shock rather than us simply rolling our eyes at old Ken Russell and his tricks.
For me, the only weak point of the film is the camp flamboyancy of the king and his 'entertainment'. It takes away from the intense claustrophobia of the nunnery and Grandier's transformation from virile priest to religious martyr- which came even more suddenly for me because the cinema projector lost the picture for a few minutes. Reed is brilliant in both roles, giving the film an epic quality though it clocks in at under two hours.