Set in a remote British boarding school, Hex is the story of Cassandra "Cassie" Hughes (Christina Cole), a shy social outcast who discovers she has unique powers that provide a terrifying link to the past.

Longing to be popular, but only truly loved by her lesbian roommate Thelma (Jemima Rooper, Kinky Boots), Cassie goes to great lengths to be accepted by the "in crowd" at her cliquish English boarding school, Medenham Hall. Thelma, who couldn't care less about being ostracized, is appalled at Cassie's attempts to fit in with the superficial classmates who treat them so badly.

While stopping for a cigarette in one of the old buildings on campus, Cassie's future is forever altered when she unearths hidden ancient artifacts - a discovery that leaves her cursed with supernatural powers that have their roots deep in Medenham Hall's history. As she begins to recognize the extent of her new abilities, Cassie is bewildered and terrified by the visions that haunt her. She soon discovers that there are certain advantages to being a student endowed with mystical powers, when she learns how to manipulate the people and situations around her.

However, Cassie realizes that her new gift comes with a price. Her tragic fate is sealed with the appearance of Azazeal (Michael Fassbender, Band of Brothers), the leader of a group of fallen angels who were banished from heaven for tasting the pleasures of mortal women. Azazeal kidnaps Thelma in a bid to get to the one human sacrifice he's really concerned with, Cassie. But when Cassie confronts him and demands to see her friend, it's Thelma who pays the ultimate price.