Zoe and Harvey are finally old enough to accompany Graham on the annual roundup of the park's wild horses. Working alongside a group of cowboys, the kids and Lassie help Graham drive the wild horses back to the valley where they can be well taken care of, and eventually placed safely on ranches in the area. When one pregnant mare goes missing, Zoe and Lassie can't leave her behind¿especially with rustlers in the area.