In the late 1850s in and around Carson City, struggles between the Indians and the local whites are growing. During the struggles, Joselyn, a young orphan, meets Sarah Winnemucca, a Paiute girl who becomes her friend and gives her some valuable advice. When Joselyn takes that advice and escapes from the Carson City Home for Unfortunate Children, she has no idea that her boy's disguise and her love for and expertise with horses will lead her straight to the Pony Express. Joselyn becomes Jo and turns to a life that demands all her inner strength and resources. Then the meanest man on the route learns her secret and uses it to extract a promise that kept or broken could mean death.