Danger lurks in the mist. Twig and Ben are unicorn riders--guardians whose job it is to keep the last free unicorn herd safe. But a new danger is threatening the beautiful, mysterious creatures of Lonehorn Island. A thief from Terracornus has snuck onto the misty island and stolen Ben's loyal unicorn, Indy. There's only one path for Ben and Twig--straight into the secret, shadowy heart of the island and through the passage to Terracornus. But their rescue mission is unexpectedly complicated by a secret Ben has been hiding. A secret about the Queen of Terracornus who has enslaved all the unicorns of Ben's homeland. A secret that could save them all--or start a war.