Hilary James (`Mousie') is sixteen when she wins The Fuller Trophy jumping with her horse Dancer at the Royal Winter Fair. Her triumph is rewarded with an invitation to perform in England for Queen Elizabeth, but she has also attracted the unwanted attention of the evil Samuel Owens who plots to acquire Dancer for his niece, Sara.
Thwarted in his initial attempt to purchase the horse, Owens has his hired man, Chad Smith, try to steal it. Mousie has a dream in which a beautiful blond horsewoman warns her of impending danger. She wakes to discover Chad Smith, syringe in hand, in Dancer's stall. Chad Smith is killed in the ensuing scuffle and his employer comes under suspicion.
Dancer is flown to Highgrove, the country home of Prince Charles, and Mousie arrives with her mother Christine at `Clusters' - an English manor, once the home of Arabella, the second wife of the Duke of Dewbury, now both long dead. Mousie finds an antique lady's hunting whip which she feels certain must have belonged to Arabella, and later discovers a portrait of her riding side-saddle. It is the same woman who appeared in Mousie's dreams.