A colt, son of Gantry the Great, a thoroughbred race horse, is born during a thunderstorm to horse trainer Mack Lowman and his son Danny. Lightning strikes the barn and sets it on fire, and although Danny successfully rescues the colt, Mack and the mare are killed. Because Mack, who had been accused of being a racetrack crook, ended his life in shame and debt, Danny hopes to grow into a prize-winning jockey with Gantry, Jr. To keep Gantry from being used to pay off Mack's debts, Danny gives him to his longtime friend, Midge Griner, whose father, Colonel Griner, runs a horse farm, and heads West. After Gantry has grown, the Griners' servant, Domino, reads that Danny, now a jockey, has been arrested for selling tips at the tracks. A Louisville judge is about to sentence Danny to reform school when Midge tells him that her father is willing to hire Danny. When the judge calls the colonel to confirm, Midge begs her father to give Danny a chance. Danny reveals that while he was on the West Coast, the horse owners found out he was Mack Lowman's son and refused to give him mounts. Later, at the Griner ranch, stablehand Dave Miller insults Danny, then whips Gantry in an attempt to train him, upsetting the horse. Although Danny immediately calms Gantry, he is deemed untrainable. Danny trains Gantry himself, and a week before the Harvard Stakes, Midge convinces the colonel to take a look at Gantry. On Griner's track, Gantry successfully wins an impromptu race against the colonel's Harvard Stakes entry, Silent Night, who pulls a tendon. The colonel then buys Gantry from Midge and Danny, promising to split the profits with them, and enters Gantry in the Harvard Stakes, with Danny as his jockey. Gantry wins and becomes Griner's only Kentucky Derby entry. During the next six months, Gantry's winnings keep the farm running. At Churchill Downs, Gantry nearly wins the Derby, but falters at the end. Danny is accused of throwing the race, but vows to exonerate himself one day. Danny then discovers that Gantry is blind, and that his blindness was probably caused by a blow that Miller gave him before quitting the farm. Although a veterinarian recommends destroying Gantry, Danny soon discovers that Gantry can run and jump as long as he is riding him. Domino helps Danny train Gantry for the steeplechase, while the colonel prepares to sell the farm and retire from racing. Domino, Danny and Gantry secretly take the boat to Liverpool, where Gantry is ruled eligible for the Grand National, which he must win to recoup the colonel's fortune. When Gantry wins, Danny pleads with the colonel during a radio broadcast to believe he never threw a race. Domino assures him that everything including the name of Mack Loman is "squared now."