On a snowy night, Cara and her grandmother are pursued into St. Christopher's church by an unknown man. Clutching her grandmother's mysterious amulet, and following the old woman's instructions, Cara escapes their pursuer and falls into Luster, the world of the unicorns. Attacked by a monstrous little creature called a "delver," Cara is rescued by a shambling, manlike beast known only as "the Dimblethum".

Soon Cara has also met a rebellious young unicorn named Lightfoot, and a ridiculous little creature called "the Squijum." With their help, Cara embarks on a journey across Luster to deliver a message from her grandmother to the queen of the unicorns. But to survive the trip, Cara must come face to face with her greatest hope, and her greatest fear.