On a wintry 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 strange instructions, Cara escapes into Luster, the Land of the Unicorns.
When a goblin-like monster called a delver tries to steal the amulet, Cara is rescued by "the Dimblethum," a huge, shambling man-beast. Soon the two new friends are joined by a rebellious young unicorn named Lightfoot, and a ridiculous creature known only as "the Squijum."
With her new friends, Cara begins a dangerous journey across Luster to deliver a message from her grandmother to the Queen of the Unicorns. Though the little band will face many perils along the way, the greatest danger of all is to Cara's heart. For to survive this trip, the girl must come face-to-face with both her greatest hope, and her greatest fear.