At an African mission, one of the teachers, Gwen Mayfield, attempts to calm her students who believe they are about to be attacked by a local voodoo priest. The students flee and Gwen is shocked into a nervous breakdown when the door bursts open and she is confronted with tribesmen wearing frightening masks.

Several months later, Gwen arrives for an interview for a Head Teacher's post at a school in the remote English town of Heddaby. Stephanie Bax and her brother, Alan, run the school. Gwen believes Alan to be a clergyman but she later discovers from Stephanie that although he wanted to enter the church he failed to qualify. Later, as he shows Gwen the ruins of what used to be the village church, he confides that the collar gives him a feeling of security.

Gwen begins teaching and becomes popular with several students, particularly Ronnie Dowsett, who is taught extra hours due to his potential to enter a top school. He tells her that his girlfriend, Linda Rigg, another pupil at the school, had her hand put in a mangler by her grandmother, Mrs Rigg, who looks after her. Gwen stops off at the grandmother's house to make enquiries, and although Linda's hand is bandaged, she claims it was an accident of her own making.

Gwen catches Ronnie and Linda arguing, and discovers Linda has lost a male doll that Ronnie gave her as a present. Later that day, Ronnie is rushed into hospital and Gwen finds the doll stuck with pins and with the head cut off. Gwen tells Stephanie of the discovery and that she suspects witchcraft. As Stephanie is a freelance journalist, she suggests they cooperate on an exposé for a magazine.

Gwen is visited by Ronnie's father, who tells her he is going to visit Mrs Rigg, now suspected by Gwen of being complicit in Ronnie's illness. The next morning he is found drowned and his body is carried through the town to be buried.

Gwen examines the lake where he was found and discovers several footprints in the mud. Before she can alert anyone, a flock of sheep, alarmed by Stephanie's dogs, stampedes over the ground and erases the markings. Gwen spends the night at Stephanie's house, and is plagued by similar dreams to those she experienced in Africa. She suffers another breakdown and is cared for in a local nursing home. She has no memory of what happened to her since she left the mission in Africa.

A chance visit from another patient's granddaughter prompts Gwen's memory, as she sees the girl carrying a doll. Gwen escapes the home and returns to Heddaby after hitching a lift with the town's butcher. While parked, she is told by a villager 'You've got to go', but nevertheless she continues back to Stephanie, who takes her in.

Gwen visits Mrs Rigg the next day to enquire about Linda, but is informed by Rigg that she is on holiday and will be back soon. She packs away a dress that she has made for Linda and claims she is posting it to her. At night, Gwen awakes to see people walking down the beach outside her window, and she follows them.

She arrives in an underground lair with a pentangle drawn on the floor. Stephanie approaches and says that she has willed her there. Gwen realises that Stephanie is practising witchcraft and the entire village is under her control. They perform a rite that makes Gwen 'one of us', and then Stephanie leads Gwen back to the house to reveal the truth about Heddaby.

Stephanie wishes to live long enough to use her power for great things, and in order to do so she is going to sacrifice Linda. Later that evening, the villagers begin the ritual, but before Stephanie can apply the final incantation, Gwen cuts her arm and spills blood over Stephanie and the altar, contaminating the area and voiding the ritual. This act kills Stephanie and breaks her hold over the villagers. Mrs Rigg embraces Linda and later, Gwen returns as Head Teacher to the school and begins a new term in Heddaby.