"Thunderhead," a roving, big white stallion, causes problems for the Wyoming ranchers when he leads their blue-blooded racing mares off to join his wild horse herd in the mountains. Escaping gunfire, he runs off one night with a young rancher;s mare, a possible winner of the Governor's Stake trotting race. The mare is recaptured and entered in the race against the horse owned by the father of the young rancher's sweetheart, and this puts a damper on their romance.