In the small, conservative town of Gatlin, South Carolina, high school student Ethan Wate is eager to escape the stifling routine of his everyday life. He longs for adventure and intellectual stimulation, both of which are scarce in his close-minded community. Ethan's world changes when he meets Lena Duchannes, the mysterious new girl who has just moved into the town's oldest and most infamous mansion. Lena is unlike anyone Ethan has encountered before: she is intelligent, literate, and harbors a family secret that sets her apart from Gatlin's ordinary inhabitants. As Ethan and Lena's friendship blossoms into romance, he discovers that she is a "Caster," a person with supernatural abilities that manifest upon her sixteenth birthday.

Lena is struggling with her upcoming birthday, which will decide her fate within the Caster world—whether she will be claimed for the powers of Light or those of Darkness. This internal battle is exacerbated by the curse that haunts her family, and by the expectations placed upon her by her powerful kin. Meanwhile, Ethan stands by Lena, drawn deeper into her enigmatic world, facing the scorn and fear of the townspeople who view the Casters with superstition and distrust. The closer Ethan and Lena become, the more they uncover about the dark secrets of their families' interwoven histories, and as Lena's sixteenth birthday approaches, their relationship may trigger catastrophic changes that could alter their lives and Gatlin forever.