When the white stallion Thunderhead steals another mare belonging to drunken rancher Beaver Greenway and his granddaughter Carey, Beaver goes to confront Rob McLaughlin, the owner of the Goose Bar Ranch. Rob, whose teenage son Ken was Thunderhead's owner before the stallion ran wild, tries to assure Beaver that Thunderhead has not been seen in several years, but Beaver storms off unconvinced. After the Greenways depart, Rob and his wife Nell welcome Ken, when he returns from a horse-selling trip. Rob is dismayed to learn that Ken met his old pal, Whitey Eaton, a disreputable character who persuaded Ken to spend his profits on a trotting horse named Crown Jewel. Rob chastises Ken for acting hastily, but Ken asserts that Rob must trust his judgment if he intends to make him a partner in the Goose Bar someday. Nell and Rob are pleased with Crown Jewel's beauty, and ranch hand Gus realizes that her flightiness is due to altitude sickness caused by the high Wyoming country. Rob agrees to let Ken keep Crown Jewel with the stipulation that he will not be made a partner until she earns back her cost. A few days pass as Ken begins Crown Jewel's training, and one afternoon, Carey teases Ken into asking her out on a date. When Ken arrives at the Greenway ranch, however, Beaver, drunk and surly, tells him that Carey does not want to see him. Fed up with Beaver, who used to be a champion trotting horse driver, Carey castigates him for his behavior and chases after Ken. The young couple have a good time at a local dance, but later that night, Thunderhead comes down from the range and lures away Crown Jewel. When Thunderhead's hiding place is spotted by a forest ranger, Ken, Rob, Beaver and a few ranch hands go in search of him. Thunderhead and Crown Jewel elude their pursuers, who nonetheless manage to round up the other missing mares. Ken spots the fugitives and eventually catches up to them in the morning, just as Thunderhead is fighting off a pack of wolves. Although Thunderhead recognizes Ken, he will not accompany him as he leads the ill Crown Jewel home. At the ranch, veterinarian Hickson asserts that Crown Jewel's lungs are so congested that she should be put out of her misery, but Ken refuses to give up on her. Ken tries everything he can think of to cure the lovely mare, but it is Beaver, who has stopped drinking and resumed training his own trotter, Sundown, who recommends a successful course of treatment. When Crown Jewel has recuperated fully, famed trainer Jake Willis agrees to train her, but her instability worries Ken. Gus points out that her love for Thunderhead is distracting her, and when the stallion again comes to the ranch, Ken succeeds in using Crown Jewel as bait to capture him. With Thunderhead by her side, Crown Jewel settles down to work, and she and Ken become a winning team. Ken and Crown Jewel enter the prestigious Governor's Cup race in Lancaster, Ohio, which is also attended by Beaver and Sundown. Carey's loyalties are torn, for she has fallen in love with Ken but knows that Beaver might start drinking again if he loses. Crown Jewel breaks her stride during the first heat and Beaver wins, but she holds true in the second heat and wins. Ken and Beaver's taut nerves are eased by Carey's encouragement, and in the final heat, Crown Jewel again breaks her stride and Beaver and Sundown triumph. After Carey accompanies Beaver on his victory lap, the Greenways visit with Ken and his parents. As Ken wonders what is ailing Crown Jewel, Beaver examines her and happily tells Ken that she faltered because she is to be a mother. Later, Carey and Ken admire Thunderlead and Crown Jewel's colt, Storm Cloud, whose white coat and blue eyes are proud reminders of his wild albino great-grandfather.