Loosely based on Prosper Mérimée's short story 'Lokis', this is far more coherent than Borowczyk's Contes immoraux, and arguably far more interesting. The American Philip Broadhurst bequeathes his estate to his daughter Lucy if she marries Mathurin (Pierre de l'Esperance's son), and is married by Cardinal Joseph do Balo. Mother and daughter make their way to the house.
We see Mathurin at the beginning of the film, watching a horse, in some detail, have sex with another. This sets the scene for the rest, and meanwhile Lucy is aroused by images of bestiality.
But although Pierre has tried to disguise the fact that Mathurin is an idiot, it becomes apparent from his table manners that something is seriously wrong. The wheelchair-bound Rammaendelo de Balo also lives in the house, and he's against the marriage as he depends on Mathurin to help him. As he attempts to dissuade the cardinal on the phone, Pierre slits his throat.
Lucy dreams of a rampant beast, wakes up and, suspicious, goes to Mathurin's room and finds him sleeping. She has another intense dream of the beast smearing his sperm over her and then dying. Lucy wakes up, goes to Mathurin's room again and finds him dead too. This is when it is discovered, on Mathurin's plaster cast on his arm breaking, that he has not a hand but a claw, that his body is thick with hair, and that he has a tail. Lucy and her mother escape.
I suppose you could call this a kind of softcore horror.