The Homestretch

The Homestretch is a 1947 American drama film directed by H. Bruce Humberstone and written by Wanda Tuchock. The film stars Cornel Wilde, Maureen O'Hara, Glenn Langan, Helen Walker, James Gleason, Henry Stephenson and Margaret Bannerman. The film was released on May 4, 1947, by 20th Century Fox.
Plot.
Jock Wallace and Kitty Brant are rival thoroughbred horse breeders. He outbids her for a promising but injured horse, Abby R, paying $500 for it, angering the owner's niece, Leslie Hale, who feels the horse is worth much more.
Jock is attracted to Leslie and invites her to the Kentucky Derby to see another of his horses run. She is involved with a diplomat, Bill Van Dyke, and plans to join him in England for the coronation of King George VI, so Jock decides to pursue her there and enter his horse in the Ascot Gold Cup instead.
On a boat to Argentina, where he intends to race Abby R next, Jock charms Leslie and they are married. But the return of Kitty, combined with Jock's broken promises and racing debts, leads to the pregnant Leslie having a miscarriage and leaving him. Valiant, a horse Jock gave her as a gift, begins winning big races at Saratoga, Belmont, Hollywood Park and other major tracks. Jock gets a gift horse from Kitty and decides to run it against Valiant in the Kentucky Derby. Leslie's horse wins by a nose, but Jock wins back her love.