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 have 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 is arrested for the murder, and it is clear that the local magistrates are happy to see him condemned without much thought. Lacey is the only person who believes the Romany innocent, and he must hurry to prove it before the young man is hanged. As Lacey investigates what the magistrates do not want him to discover, the intrigue surrounding the murder becomes as murky as the waters of the canal itself, putting Lacey and Grenville into serious danger.