Orphaned at age six and taken in by a heartless couple, nine-year-old Katie Rose spends her days doing chores and dreaming of going west to find her Uncle Jack. Then Mr. Stevens brings home an unbroken Mustang, and Katie's world changes. Katie is drawn to the horse's wildness, and he seems to sense her need for companionship. So when Katie learns that the Stevenses plan to join the expansion West, without her or the Mustang, she makes a desperate decision to go on her own. And she will not leave the Mustang behind.