The Unicorn Thief

"Twig and Ben are hard at work taming the last free unicorn herd on Lonehorn Island when Ben's loyal unicorn, Indy, disappears. When their search leads them to the island's secret passageway of Terracornus, the land of unicorns, they discover a bold thief at work. There Twig learns the tangled truth about Ben and the evil queen who's enslaved all of the unicorns. Now all of the captive unicorns are in danger and only Ben and Twig, the last unicorn riders, can save them"--