Head trainer John Mason from Shoscombe Old Place, a racing stable in Berkshire, comes to Sherlock Holmes about his master, Sir Robert Norberton, Baronet. Mason thinks he has gone mad. Sir Robert's sister, Lady Beatrice Falder, owns Shoscombe, but it will revert to her late husband's brother when she dies. The stable has a horse, Shoscombe Prince, who Sir Robert hopes will win the Derby. He would be out of debt if that actually happened.
Mason is not quite sure what he wants Holmes to investigate, but a number of odd changes have happened at the stable: