Captain Lacey takes a post as a secretary at the Sudbury School in Berkshire, a school for sons of the wealthiest merchants and bankers in England. He discovers as soon as he arrives that he's been hired for more than his letter-writing skills—a series of disturbing pranks has kept the school in an uproar. The headmaster expects Lacey to discover the identity of the prankster—immediately.
Problems intensify when a groom of the school's stables turns up dead in a lock of the nearby canal. A Romany whose family lives on a canal boat is arrested for the murder, and Lacey is the only person who believes the young man innocent. As Lacey races to solve the puzzle before the Romany is hanged, Marianne turns up to entrust Lacey with her greatest secret. Meanwhile, the intrigue surrounding the murder and the pranks at the school becomes as murky as the waters of the canal, plunging Lacey and Grenville into serious danger.