Jock Wallace, scion of a well-known Maryland horse breeding family, and socialite Kitty Brant are longtime rivals at the horse racing tracks, but enjoy a close, bantering relationship. When the owner of racehorse "Abby R," which has recently won a major race at Santa Anita, dies suddenly, Jock and Kitty compete to buy the horse. In Boston, Jock sees the late owner's sister-in-law and pays $500 for the horse. Later, however, her niece Leslie Hale, accuses Jock of not paying enough and demands another $500. She is amazed when he tells her that the horse is actually worth about $30,000. Jock invites Leslie to see Abby R win the Kentucky Derby, but she informs him that she will be in London, where she is to marry U.S. State Department employee Bill Van Dyke III. Very interested in Leslie, Jock decides to enter Abby R to run at Ascot in England. The Coronation of King George VI is taking place in London, and when Bill has to leave suddenly to attend to embassy business in Madrid, Jock offers to escort Leslie to a dinner and a ball. The next day, she accompanies Jock to watch Abby R in practice runs and meets Jock's trainer, Doc Kilborne. Soon she is a regular attendee at the horse's workouts and is falling in love with Jock, who asks her to go with him to Argentina, where he plans to race Abby R after Ascot. Bill returns, however, and now compelled to choose between them, Leslie elects to continue with Bill. During the big race, Abby R has trouble with a leg and loses. Jock and Doc are about to leave on an Argentina-bound ship when Leslie shows up with a present for Jock--"Valiant," a colt he has admired. Having decided to marry Jock, Leslie sails with him to Argentina. In Buenos Aires, the trio is welcomed at a party attended by several of Jock's horse racing friends, including Kitty, with whom Jock immediately begins competing to buy some local horses. Meanwhile, Doc treats Abby R's leg and soon has the horse running better than ever. When Leslie, who has discovered that Jock has been borrowing money from Kitty to support their lavish lifestyle, becomes pregnant, she begs Jock to take her back to his home in Maryland. Jock tells her that if Abby R wins the Gran Premio race, they will be able to return and also have enough money to repair the property. However, after Abby R wins, Jock goes back on his word and does not plan to return immediately to the U.S. Meantime, he and Kitty are still seeing each other, and after finding them together one evening, Leslie drives off in a rage and crashes into a tree. She loses the baby and decides to divorce Jock, accusing him of never having wanted the baby. She has sent for Bill and together they fly back to the U.S. Back in Boston, Doc visits Leslie and asks her what he should do with Valiant, as Jock has signed the horse back to her. As a form of revenge, Leslie decides to race Valiant in the Kentucky Derby against Relampago, a horse Kitty has bought in Argentina, and she and Doc try out Valiant at Hialeah, where Relampago is also running. Relampago is virtually uncontrollable and throws his jockey for the third time. Valiant wins and also triumphs against Relampago in races at Saratoga, Belmont, Jamaica and Hollywood Park. Meanwhile, Jock has returned to the U.S. and is working at Hollywood Park where, after another disastrous showing, Kitty gives him Relampago. After Jock discovers that the horse is only happy and malleable when spoken to in Spanish, he enters Relampago in the Kentucky Derby in direct competition with Valiant. Doc tells Leslie that Jock has staked everything, including his farm, on winning, and when the race ends with a neck-and-neck finish between Valiant and Relampago, Leslie finds herself cheering on Relampago, to Bill's dismay. Valiant wins, but back in Maryland, Jock discovers that Doc has returned with both Relampago and Valiant as well as Leslie, who has finally decided to spend the rest of her life with Jock.