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After jockey "Tip" Collins, riding Avalanche, the horse in the lead in the Melbourne Sweepstakes, fouls another rider, Avalanche is disqualified. Major Gordon Kent, who gave Avalanche as a wedding present to the internationally known American sportsman George Chester when Chester married his daughter Catherine, believes that a big gambling ring is behind the foul. The major has a wire sent to his old friend, the renowned detective Charlie Chan, instructing him to meet their boat in Honolulu on their way to compete in America. However, on the voyage, the major dies seemingly from being kicked by Avalanche in his stall. Chan determines from the position and the shape of the bloodstains that the horse could not have kicked the major. After Chan reveals to his chief and the ship's captain a piece of the ship's winch, the twin of which is missing, which could make a shape identical to that of a horseshoe, the chief suggests that Chan travel with the boat to investigate what they now suspect is a murder. When Chester receives a typed note warning him not to enter Avalanche in the Santa Juanita Handicap, Chan's son Lee, who, against his father's wishes, got on the boat as a cabin boy, determines that the note came from the typewriter of the major's competitor, Warren Fenton. A number of other passengers next receive notes: Fenton, who offered Chester $20,000 for the horse; Bruce Rogers, the major's assistant, who is in love with Fenton's daughter Alice; Denny Barton, who also loves Alice, but whom she has rebuked; and Chester again. After a fire breaks out in the forward hold where Avalanche is kept, Chan is hit in the leg by a bullet fired accidentally by Chester. In Los Angeles harbor, Chan notices that a monkey, who earlier caused Avalanche to bolt, now causes Fenton's horse Gladstone to go wild, while Avalanche does not mind the monkey. Chan suspects that the fire was used as a cover so that the horses could be switched and Fenton's horse could then win the upcoming race with good odds. The switch, involving the application of black dye to Gladstone, was engineered by Avalanche's trainer Bagley working with a gang of gamblers. On the day of the race, Lee creates a diversion so that Chan can enter the stables and switch the horses. Bagley, after noticing the switch, is arrested as he calls a gambler. As the race begins, Al Meers, a track employee in league with the gamblers, switches a device at the three-quarter pole, which is used to time the race, with one fitted with a dart. As Avalanche, in the lead, passes the pole, the dart hits the horse. Avalanche wins anyway, but then falls. As a crowd surrounds Avalanche, someone removes the dart. Chan then gathers Denny, Bagley, Meers, Chester and Fenton in the racing association office. When the dart is found in Fenton's pocket, Fenton accuses Denny of putting it there, but Chester accuses Fenton of wanting to buy Avalanche all along and of murdering Major Kent with the winch shoe because the major would have noticed that the horses had been switched. Chan then points out that no one other than himself, his chief, the captain of the ship and the murderer knew about the winch shoe. He says that he suspected Chester all along because Chester, who admits that he suffered gambling losses, did not use his glasses to read the first threatening note he received, which Chester himself sent to throw off suspicion, but that he did use his glasses to read the second note, which Chan, with Lee's help, sent. Chan then reveals blood stains from the dart in the lining of Chester's pocket. Fenton confesses that he knew of the plot to switch horses and tells the commissioner that he will remove his horses from the track. Bruce wins enough money from the race to furnish a flat for himself and Alice. When the ever-enthusiastic Lee pops in with what he thinks is a hot clue, Chan requests that he save it for their next case.