Claire Martinson still worries about her older sister Sophie, who battled a mysterious illness last year. But things are back to normal as they move into Windermere Manor... until the sisters climb a strange ladder in a fireplace and enter the magical land of Arden. There, they find a world in turmoil. The four guilds of magic no longer trust each other. The beloved unicorns have gone, and terrible wraiths roam freely. Scared, the girls return home. But when Sophie vanishes, it will take all of Claire's courage to climb back up the ladder, find her sister, and uncover the unicorns' greatest secret.

Claire Martinson and her sister Sophie have decided to stay in Arden—the magical land they discovered by climbing up a chimney in their great-aunt's manor. If what they've learned is true, the sisters are the last descendants of the royal family, and only a true heir of Arden—with magic in her blood—can awaken the unicorns. Since Sophie has does not have magic, the land's last hope rests on Claire. The sisters journey to Stonehaven, a famed Gemmer school high in the mountains of Arden, so Claire can train in the magic of stone. As Claire struggles through classes, Sophie uncovers dangerous secrets about the people they thought they could trust. With Arden on the brink of crumbling, can Claire prove she is the prophesied heir and unlock the magic of the unicorns before it's too late?

After a lifetime of being the younger sister—of letting Sophie fight her battles, of following her on countless Experiences and through a fireplace into a magical land—it's finally Claire Martinson's turn to lead. And she'll do anything to keep her big sister safe: resist the wraiths' cold shadows, experiment with new and dangerous magic, become a thief in disguise. When Claire discovers that "only a queen can defeat a queen," she knows she must steal and reforge the ancient Crown of Arden to stand a chance against the darkness that threatens the world and her family. Because Queen Estelle d'Astora, desperate to reclaim her power, will stop at nothing to gain the support of the four guilds—even if it means killing the last unicorn. Claire will need every friend she's made to help her as the fate of Arden hangs in the balance . . . But the secrets of the unicorns are deeper than anyone could have ever imagined. Does Claire have what it takes to ignite the long-buried magic of this world and wake the fire in the stars? Or will the true salvation of Arden rest on one final and heartbreaking sacrifice?