Hex is set at a remote English school, Medenham Hall, which occupies the former manor house and grounds of the McBain estate. It was the site of an 18th century witchcraft scandal in which the female members of the line, the Medenham Witches, were tried and executed. Cassie, who raised herself in a single-parent household caring for her mentally unstable mother, has latent telekinetic and clairvoyant abilities that are awakened when she touches an antique vase that had been used in Voodoo rituals by the Medenham Witches. Her roommate, Thelma (who harbors an ill-concealed crush on Cassie), discovers that Cassie is a descendant of the Medenham Witches. Cassie has startling visions and dreams that she strives to interpret, and also struggles with controlling her growing telekinetic powers, which she is only able to summon in times of stress.