Penny Simms is pony-mad and is thrilled when she wins a scholarship to Fetlocks Hall, an equestrian boarding school. But Fetlocks Hall is no ordinary school - besides schoolwork and riding lessons, Penny has the opportunity to acquire secret knowledge about horses as well as find out about the magical unicorn world of Equitopia. Soon she is caught up in a desperate struggle to help the unicorns combat a terrible plot by the evil Devlipeds, who are plotting to overthrow them. Is Penny the Unicorn Princess whose arrival is predicted by the King of the Unicorns? Can she defeat the Devlipeds and save the unicorns and the pony world?