In the quiet town of Gatlin, South Carolina, Ethan Wate awakens from a vivid recurring dream involving a mysterious girl he cannot identify. Through his introspective voice-over, he shares his passion for reading forbidden literature and expresses his frustration with his mundane small-town life, yearning for a future beyond. On his first day of junior year, Ethan spots the new girl, Lena Duchannes, who strikingly resembles the girl from his dreams. Unfortunately, her arrival sparks hostility, as her classmates whisper about her secluded uncle, Macon Ravenwood, and speculate about her family's rumored devil-worshipping practices. Feeling the weight of the gossip, Lena causes the classroom windows to shatter, feeding into the town’s fears that she’s a witch.
Later, while driving home, Ethan nearly hits Lena when her car breaks down. Offering her a ride, they connect over their mutual love for poetry and the shared pain of having lost their mothers. After dropping her off, Ethan finds a locket and decides to return it as a gift. Upon touching the locket, they experience a shared flashback to the Civil War era, only for Ethan to awaken back home. Meanwhile, Macon seeks to intervene in their blossoming romance, collaborating with Ethan's family friend, Amma, to keep them apart. Undeterred, Ethan pursues Lena, who eventually reveals the truth about her lineage as a "caster" capable of various forms of magic, including weather manipulation and illusion crafting. With her sixteenth birthday approaching, Lena is torn between the power of light and the danger of embracing darkness – a fate that could lead her to hurt the ones she loves. Ethan encourages her to take charge of her own destiny, assuring her that she is inherently good.
However, complications arise with the appearance of two formidable dark casters: Ridley, Lena's alluring cousin and childhood friend, alongside Sarafine, Lena's mother, who has taken over the body of Ethan's friend Link's mother. Sarafine envisions Lena fully mastering her powers and intends for her to eradicate humanity, ensuring that casters dominate the world. Lena and Ethan use the locket to delve deeper into their shared past, uncovering a love story between Lena’s ancestor, Genevieve Duchannes, a caster, and Ethan Carter Wate, a mortal Confederate soldier. Their love was tragically cut short when Ethan was killed in battle, leading Genevieve to use forbidden magic to revive him, resulting in a curse that befell all of the Duchannes women.
As they seek the truth, they consult with Amma, who serves as a seer and keeper of a caster library hidden beneath the town library. There, they discover the Book of Moons, which reveals a devastating secret: to break the curse, a loved one must die. Torn apart by what this means, Lena chooses to erase Ethan's memories of their time together rather than sacrifice him. Meanwhile, Ridley entices Link, giving him a bullet for a reenactment of the Battle of Honey Hill scheduled on Lena’s birthday. During this event, Link and Ethan plan to "kill" each other to escape, unaware of the chaos about to unfold.
On her birthday, Lena feels the curse’s power break, prompting her to rush to Ethan, only to find him fatally wounded. As Ridley and Sarafine urge her to embrace her grief and surrender to darkness, Lena erupts in anger, unleashing lightning throughout the reenactment crowd until Ethan physically transforms into Macon, who had taken Ethan’s place as the necessary sacrifice. In his final moments, Macon urges Lena to “claim yourself,” leading her to cause the moon to vanish, preventing its influence from pulling her into darkness. After sparing Ridley, Lena confronts Sarafine, finally sealing her spirit away.
Months later, a still-amnesiac Ethan visits the library and offers an apology to Lena for their lost opportunities. She is found in the caster library, having accepted her identity as a caster of both light and dark. As Ethan drives away with Link, memories of Lena gradually flood back, prompting him to call out her name. The story concludes with Lena hearing his voice, hinting at the possibility of their reunion.
