what are the twists and turns of this film?

A lot of it concerns talking pussy. Pretty straightforward stuff.

A girl, Catherine, and her parents are going to visit an uncle (the father's brother if I remember correctly). The father runs into a tree knocking the mother unconscious. Catherine leaves the car to get help when she sees it burst into flames behind her, leaving both of her parents dead. She stays with her uncle, her cousin (Stephen) and the uncle's secretary, Francis. It is mentioned Catherine has premonitions so she has visions relating to witchcraft and devil worship. She falls in love with her cousin and wants to leave somewhere with him, but the uncle has other plans.

On the night of Catherine's 20th birthday, they plan to sacrifice her to resurrect an ancestor named Camilla Yorke (who was said to have evil powers) who died when she was 20. Frances tries to help her escape from this sacrifice but Stephen finds out and kills her. When it is time for the sacrifice, she is brought into the woods where there is a coven in robes. There is a struggle where she kills Stephen and barely escapes from the woods. She runs into the arms of her father who she thought to have died in the exploding car. He tells her that during the car accident she hit her head and was knocked out for days. She is confused and believes everything that happened after the car wreck was a dream. They bring her back into the house where her uncle tries to comfort her and suggests she call her boyfriend (who they earlier told her was dead) back home on the phone which is "behind the curtain." When she moves the curtain, she sees Stephen's dead body with the knife in his eye and knows it wasn't a dream. The cult (with the help of Catherine's father) sacrifices her anyways.

This is all from memory so I may be forgetting a detail or two, but this is all the important stuff that happened in the film.