A group of gangsters steal the valuable brood-mare "Larkspur" from the Conway Ranch in Arizona. The mare's three-month-old colt tries to follow its dam, but is outdistanced. The lost colt finds its way to the Carey farm, where David Carey, a 16-year-old orphan, lives with his miserly uncle Thad Carey and his aunt Hildy Carey. David adopts the colt and names it "Sunset." When the colt isn't filling out as David thinks it should be, he goes to the Conway Ranch to consult the head groom, Jack Lenihan. When Bill Conway and his young daughter, Wendy, return to the ranch, David learns that his colt is "Larkspur's" foal, and belongs to the Conways. David goes home and tries to return to colt, but "Sunset" won't leave, and David decides it's a matter of finders-keepers. But the colt breaks a leg and may have to be put down, unless a newly-invented trick of putting a dental-cement cast on a horse's broken leg works for veterinarian 'Doc" Purdy.