Henry Kendall likes the timing at practice, so he enters his horse, Take A Chance for the Gold Cup. Wife Enid Stamp-Taylor has a snit, walks out for an afternoon, and informs her lover that the horse will win. This causes the odds to drop from 50 to one down to the single digits. The bookmakers go crazy as they fail to lay off their bets. Finally, they turn to handicapper (an author of a "tout-sheet" con) Claude Hulbert to try to get the horse scratched. There also follows a bunch of complications as Kendall gets wind that the information is loose and considers scratching the horse on his own.
It's a well-performed if not particularly excellent farce, with Binnie Hale and Jack Barty getting involved in the shenanigans, leading to the inevitable race and the question about whether the horse will win. In America, the screwball comedies of the period confused money and class. Perhaps this attitude of the British film makers is more realistic, but realism doesn't go far in making an amusing farce.
Perhaps I have seen too many comedies and dramas about horse racing for this to have much of an impact. For me, the humor comes from watching the men in their Bond Street clothing and the ladies in their furs, moving in posh settings, and utterly obsessed about the money. Only Kendall seems to have much interest in horse racing as a sport. Perhaps that take is peculiar to me; I have little interest in the question of whether one horse is faster than another at a particular moment.