Libby wants a horse more than anything in the world. Since she is unable to have one of her own, she's been doing stable chores for her neighbors, the Porters, for the past three years in exchange for riding lessons from Jolene Porter. Libby forms a special bond with the Porters' prize Appaloosa, Thunder, but this arrangement comes crashing to an end when Jolene abruptly disappears. With Jolene gone, Mr. Porter refuses to let Libby visit Thunder any longer. Making matters worse, she soon discovers that he's taking out his anger on his animals. With the help of Griff, a new boy in town, Libby devises a daring plan to steal Thunder. But how long can they stay on the run and keep Thunder safe, when Mr. Porter holds all the power?