In an icy castle in Faerie, FIACHRA, prince of Faerie's Winter Court, meets with his father. The king blesses Fiachra's quest to claim a bride from Earth, not knowing Fiachra plans to usurp his throne after the wedding.
ARIEL HAWK, an outspoken sixteen-year-old with a well-founded belief in Fae, wants to enjoy a safe, normal Halloween with her friends. As she and her friends leave a party in Leeds, they talk about the Fae; Ariel's friends mock her beliefs, while Ariel argues that the Fae exist. Fiachra, having followed them and overheard, claims Ariel as his bride, since her belief makes her ideal in his eyes. Ariel and her friends fend Fiachra off, then flee. Ariel phones her dad, VINCENT HAWK, for help. Fiachra sedates Ariel and takes her to Faerie, and Vincent is too late to stop him. To find Ariel, Vincent and his brother DOM HAWK enter Faerie via a portal in Stonehenge.
In the king's castle, Fiachra brings Ariel to meet the king. To her dismay, the king approves of Fiachra's choice of a bride. Ariel attends a feast in honor of her and Fiachra's engagement, and defies Fiachra's attempts to trap her in Faerie with enchanted food. When Fiachra tries to force-feed her the next morning, she jams her finger in his eye to defend herself, and is whipped as punishment. Fiachra's sister MÉABH comes to Ariel's aid, to take revenge on her brother for his betrayal of Méabh and her lover years ago. Méabh promises to help Ariel by finding Vincent, whom Ariel is sure has entered Faerie to help her.
Ariel befriends some kindly Fae, but gets impatient with waiting for news from Méabh, and flees the castle against Méabh's advice. Fiachra catches her and locks her in a dungeon, but she breaks out and escapes into the woods surrounding the castle. There she meets a Fae who saw Vincent elsewhere in Faerie a few days ago, confirming Ariel's hope.
Ariel is recaptured by the castle guards and returned to the castle. Fiachra rushes her through the preparations for the wedding, weeks earlier than he had said at first. She tries to stall him, saying they can't marry since she's too young, and they don't have her dad's permission. Fed up with her resistance, Fiachra tries to kill her instead—he claims she's more trouble than she's worth, and he can become king without her. She fights back and puts out his eye in self-defense. Despite her pleas to the contrary, the king calls it attempted regicide. He orders her execution for the next morning, calling it less of a hassle than returning her to Earth. Ariel loses hope of escaping or being saved.
That night, Ariel's Fae friends help her break out of her cell and flee into the woods, where they find Vincent, Dom and Méabh. The king and his guards track the group down and surround them, demanding Ariel's return. Rather than fight, Vincent tries to convince the king to let Ariel go, saying death is too harsh a penalty for Ariel's crime. Fiachra, who had followed in secret, stabs the king before he can respond. A guard then kills Fiachra, while Vincent, Dom, Ariel and Méabh rush to the king's aid. They succeed in closing his wound.
In return for saving his life, the king lets the humans leave Faerie. Ariel says goodbye to Méabh and her Fae friends, and the humans return to Earth. Ariel reunites with her human friends, and tries to readjust to a normal life.