In Gatlin, South Carolina, teenager Ethan Wate awakens from a recurring dream of a girl he has not met. He despairs of his small-town existence and dreams of leaving for college. His first day of junior year, a newcomer Lena Duchannes resembles the girl Ethan has been dreaming about. Other students gossip about Lena's reclusive uncle, Macon Ravenwood, suggesting her family are devil worshippers. On the way home, Ethan nearly runs her over. Giving her a ride, they bond over interests.
In class, a few students insist they can not be in class with Lena, and pray to be protected from her and her family. Visibly shaken, her class windows shatter, increasing fears and suspicions she is a witch. Ethan and Lena become friends and he gives her a locket he found at Greenbriar. Both touching the locket triggers a shared flashback to the Civil War.
Macon, disapproving of their relationship, conspires with Ethan's family friend, Amma, to separate them. Lena tells Ethan that she and her family are "casters" capable of performing spells. On her sixteenth birthday, her true nature will reveal itself toward either the light or dark.
Complications arise when two powerful dark casters aim to push Lena to the dark: Ridley, Lena's cousin, and Lena's mother, Sarafine, who did not raise Lena and has possessed Mrs. Lincoln, the mother of Ethan's friend Link.
Sarafine foresees Lena will become a powerful caster, wanting Lena to use her power to purge the Earth of humans, leaving just casters. The couple have another flashback of their past with the locket, revealing that their ancestors, caster Genevieve Duchannes and mortal Confederate soldier Ethan Carter Wate, were in love. Ethan died in battle, and Genevieve revived him. Doing so laid a curse on all Duchannes women; they will go dark on their sixteenth birthday. A mortified Lena asks Amma to help, being a seer and keeper of the caster library beneath the town library, extending all across the USA. The most ancient of the books reveals the secret to undoing the curse: someone Lena loves has to die. Unwilling to take Ethan's life, Lena erases his memories of their love.
Seducing Link, Ridley gives him a bullet to use in the upcoming Civil War reenactment of the Battle of Honey Hill, also Lena's sixteenth birthday, Lena feels the shock of the curse being broken and runs to Ethan, clutching his dying body as Ridley and Sarafine encourage her to accept the dark. She lashes out in anger, sending a huge tornado through the crowd, until Ethan's body transforms into Macon, who had disguised himself to be the sacrifice to lift the curse. As he is dying, he reveals that he promised Ethan's mother to keep her son alive. These dying words encourage Lena to "claim [her]self"; she causes the moon to disappear so it cannot claim her for the dark. Allowing Ridley to flee, she pulls Sarafine from Mrs. Lincoln's body, powerfully sealing away Sarafine's spirit.
Six months later, a still-amnesiac Ethan visits Amma in the library before a college tour with Link. Lena gives him a book they had once shared as a present. Lena is a half light/dark caster, like her mother and cousin. As Link and Ethan reach the town line, the town's burned exit sign reminds him momentarily, he gets out of the car, shouting Lena. She hears his call and is freed of her dark side.
Alden Ehrenreich as Ethan Wate
Alice Englert as Lena Duchannes
Jeremy Irons as Macon Melchizedek Ravenwood
Viola Davis as Amarie Treadeau, "Amma"
Emmy Rossum as Ridley Duchannes
Thomas Mann as Wesley Jefferson Lincoln, "Link"
Emma Thompson as Mavis Lincoln/Sarafine Duchannes
Margo Martindale as Delphine Duchannes, "Aunt Del"
Eileen Atkins as Emmaline Duchannes, "Gramma"
Zoey Deutch as Emily Asher
Tiffany Boone as Savannah Snow
Kyle Gallner as Larkin Kent
Rachel Brosnahan as Genevieve Katherine Duchannes
Pruitt Taylor Vince as Mr. Lee
Robin Skye as Mrs. Hester
Randy Redd as Reverend Stephens
Lance E. Nichols as Mayor Snow
Leslie Castay as Principal Herbert
Sam Gilroy as Ethan Carter Wate
Gwendolyn Mulamba as Mrs. Snow
Cindy Hogan as Mrs. Asher
based on the 2009 novel of the same name by Kami Garcia and Margaret Stohl.