Something strange is happening to Mr. Fetlock's well-trained racehorses! Suddenly his swiftest steeds are running slowly. Sometimes they just sit down. Nine-year-old Harriet Bean is just the right size to pose as a jockey and unravel this bizarre mystery. But as Harriet sits next to Black Lightning, she has second thoughts about riding such a terrifying beast and exposing the diabolical League of Cheats.
