A story of adventure and self-discovery. Each book in the Mustang Mountain series is a thrilling read filled with stories of friendship, adventure and intrigue. In the sixth book of the fast-paced series, Becky, Meg and Alison return to the ranch deep in the Rocky Mountains for summer vacation. Becky is thrilled to be home, and even Alison's annoying ways don't bother her too much. But Alison goes too far when she sends Thomas Horne on a wild goose chase after a phantom horse named Diablo. At first, she had scoffed at the ranch cook's ghost stories of a horse that haunts nearby Rainbow Valley. But as the days pass and Thomas fails to appear, everyone begins to worry that something terrible must have happened to him. As the girls set out with a search party, they soon realize he may not be the only one in danger. Can they avoid the curse of the ghost horse and make it back to Mustang Mountain Ranch safely?