Hex Hall follows Sophie Mercer, a sixteen-year-old witch raised by her human mother, who has never met her warlock father.
Sophie's troubles begin at her high school prom when she casts a love spell on behalf of Felicia Miller, a bullied classmate. The spell works too well: The target, Kevin Bridges, crashes his car through the gymnasium doors and becomes violently obsessed with Felicia. In the chaos, Felicia identifies Sophie as a witch. This is the latest in a string of magical incidents, and Sophie's father, working through the Council, the governing body of all Prodigium (supernatural beings including witches, shapeshifters, and faeries), sentences her to Hecate Hall, a reform school for young Prodigium on the remote Graymalkin Island off the coast of Georgia.
Sophie arrives with her mother, Grace Mercer, a human who teaches religious studies. Her parents split before she was born after her father revealed his true nature, and they communicate only by e-mail and phone. On the lawn, a young werewolf charges Sophie, and a warlock named Archer Cross deflects the attack with a blocking spell. The headmistress, Mrs. Anastasia Casnoff, introduces Sophie to the school's origin story: Prodigium are descendants of angels cast out of heaven for refusing to choose sides in the war between God and Lucifer.
Sophie rooms with Jenna Talbot, the school's only vampire student, admitted under a controversial reform program. Sophie warms to Jenna, who wears a bloodstone, a charm filled with witch blood that protects her from sunlight. The two begin a cautious friendship, though Jenna grows tense when Sophie mentions her previous roommate, Holly. Three dark witches, Elodie Parris, Anna Gilroy, and Chaston Burnett, try to recruit Sophie into their coven. They explain that dark witches possess far stronger powers than white witches, including necromancy, and that Sophie is the only other dark witch at Hecate. They claim Jenna murdered Holly, who was found drained of blood with puncture wounds in her neck, though a Council investigator ruled the wounds too neat to be vampire bites. When Sophie refuses, Elodie warns that without their protection she faces social ruin, partly because of something about her father they will not explain.
On the first night, Mrs. Casnoff traumatizes students with lifelike projections of Prodigium murdered by humans throughout history. She introduces three threat groups: the Alliance, a government agency; the Brannicks, an Irish family of witch hunters; and L'Occhio di Dio (The Eye of God), an elite group of assassins based in Rome. During the display, Archer holds Sophie's hand, and an intimacy develops between them.
In Defense class, taught by the stern Ms. Vanderlyden (the Vandy), Sophie follows bad advice from Elodie and compliments the Vandy's tattoos, which are marks of the Removal, a ritual that stripped the Vandy's powers. Sophie earns cellar duty for the semester; Archer protests and is assigned to share it. Archer then reveals that Sophie's father is the head of the Council, the person who sentenced every student to Hecate and authorized the Vandy's Removal. Elodie deliberately set Sophie up. Mrs. Casnoff later reveals deeper family secrets: Sophie's great-grandmother, Alice Barrow, was a human transformed through a black magic ritual in 1939 into the most powerful witch ever recorded. That power passed through the bloodline to Sophie's grandmother Lucy, to her father, and to Sophie. Both Alice and Lucy were killed by L'Occhio di Dio, and Sophie's father sent her to Hecate partly for protection.
Sophie and Archer develop a genuine friendship during cellar duty, talking rather than cataloguing enchanted artifacts that rearrange themselves between visits. Sophie recognizes her feelings are deepening. Meanwhile, a ghost girl in a green cardigan repeatedly appears to Sophie, staring at her and vanishing.
The attacks escalate. Sophie finds Chaston in a bathtub, barely alive, with puncture wounds mirroring Holly's death. Cal, the school's young groundskeeper and a powerful healer, saves Chaston. Elodie accuses Jenna, and Jenna fires back, revealing the coven had been trying to raise a demon. Anna then steals Jenna's bloodstone while she sleeps, nearly killing Jenna when morning sunlight burns her skin. Sophie retrieves it with a locator spell, but Jenna refuses to report the theft.
One night, the ghost girl puts the school to sleep and reveals herself as Alice Barrow, Sophie's great-grandmother. Corporeal and powerful, she leads Sophie to a hidden cemetery in the forbidden woods, where a stone angel holds a sword of demonglass, a substance forged in hell and the only thing that can kill demons. Alice trains Sophie in advanced magic and gives her a necklace with a black stone, a pouch of grave dirt that enhances spellcasting, and a protective mark on Sophie's left hand. She warns that L'Occhio di Dio can see Sophie even at Hecate.
Before the All Hallow's Eve Ball, Sophie discovers Elodie cursed her dress dummy. In retaliation, she plants an enchanted bone from Alice in Elodie's dress, believing it will only change the color. At the ball, Archer tells Sophie he wanted to spend the evening with her, and they dance. A scream interrupts: Anna has been attacked, found with puncture wounds in her neck. Mrs. Casnoff expels Jenna and the vampire teacher Lord Byron, concluding vampires are responsible.
Elodie catches Sophie sneaking out to meet Alice and, lured by the promise of powerful new magic, begins joining the nightly sessions. Alice shows them the wound where her heart was cut out, claiming the Council's head betrayed her to The Eye. During a charged argument in the cellar, Sophie kisses Archer. As they embrace, the protective mark on her left hand burns against his chest, revealing a hidden tattoo: a black eye with a golden iris, the mark of L'Occhio di Dio. Archer is an Eye agent. Sophie fights him off, and he escapes through the cellar window. Search parties find no trace of him.
Jenna is cleared and returns to Hecate. When Sophie retrieves Jenna's stuffed lion from the closet, Jenna recoils: The necklace Alice gave Sophie contains demon blood, causing Jenna's bloodstone to agitate violently. Sophie realizes the ritual did not make Alice a witch; it made her a demon. The coven's earlier summoning ritual pulled Alice from hell, and she has been feeding on dark witches to regain her strength.
Sophie tells Jenna to fetch Mrs. Casnoff and Cal, then races into the woods, where Elodie is practicing with Alice. Alice senses rescuers approaching, paralyzes Sophie against a tree, and drinks Elodie's blood. As Elodie lies dying, she takes Sophie's hand and transfers her remaining magic. Sophie uses the combined power to teleport in front of Alice, breaks the demonglass sword from the angel statue, and cuts off Alice's head. Alice dissolves into the earth, but Elodie is dead.
Mrs. Casnoff explains that the demonic nature passed through the bloodline: Sophie's grandmother, her father, and Sophie herself are all part demon. Lucy lived thirty years before reverting and killing Sophie's grandfather, and Sophie faces the same risk. She asks about the Removal and learns it would almost certainly kill her. When Sophie's mother arrives to take her home, Sophie sees Elodie's ghost wandering the halls and announces she will not leave. Instead, she intends to travel to London and undergo the Removal, choosing the risk of death over the certainty of eventually becoming a danger to those she loves.