The Duke of Holdernesse needs Holmes's help when his son, Lord Arthur Saltire, is kidnapped from the Priory School. Holmes finds the connection between an old mystery and a teacher who has also gone missing.

Watson, on holiday in the Lake District, meets the enigmatic Lady Frances Carfax, who is being haunted by the ghost of a horseman. Holmes decides to come up to join Watson, but by the time he arrives Lady Frances has vanished without trace. Holmes investigates and becomes very curious about the ghost.