When a guest at Colorado's Half Moon Ranch becomes injured while riding a horse named Johnny Mohawk, thirteen-year-old Kirstie must prove the horse's innocence to prevent a costly lawsuit.

At Kirstie Scott's urging, her mother Sandy buys an abused mustang stallion to save him from the brutal treatment he has received in a rodeo, but it is far from clear that Rodeo Rocky will be able to adjust to life as a dude ranch horse.