Bonnie Montgomery (as her case-cracking alter-ego Montgomery Bonbon) has been invited to Adderstone Manor by Fergus Croke, whose prized diamond has gone missing. Croke is a retired master thief who suspects one of his four protégés of the dirty deed - but when everyone is a crook, how does even the great Bonbon know where to begin?
The plot thickens faster than a good roux when the only unsuspicious member of the household, Croke's loyal butler, is found dead in the grounds - and it's not long before Bonbon uncovers hidden identities, motives a-many, and a rather surprising hedge maze full of monkeys. Can Bonbon, Dana and Grampa Banks uncover the missing gem and catch the thief and killer - and are they one and the same?
