Lucinda, Millie, Kate and Tom are on holiday at the little coastal town of Wild Horse Beach and discover a legend that tells of children disappearing from the town over hundreds of years. According to the legend, wild horses have been seen on the beach at the time of each disappearance. A snap decision by Lucinda makes the legend real and the children travel through a magic gateway into a terrifying world where dragons and unicorns need human children to help them battle frightening enemies. The children cannot leave this world without permission from the unicorn king, but that permission has a price.