On 5 September 1785, a trial began in Paris that would divide the country, captivate Europe and send the French monarchy tumbling down the slope towards the Revolution. Cardinal Louis de Rohan, scion of one of the most ancient and distinguished families in France, stood accused of forging Marie Antoinette's signature to fraudulently obtain the most expensive piece of jewellery in Europe - a 2,400-carat necklace worth 1.6 million francs. Where were the diamonds now? Was Rohan entirely innocent? Was, for that matter, the queen? What was the role of the charismatic magus, the comte de Cagliostro, who was rumoured to be two-thousand-years old and capable of transforming metal into gold? This is a tale of political machinations and extravagance on an enormous scale; of kidnappings, prison breaks and assassination attempts; of hapless French police disguised as colliers, reams of lesbian pornography and a duel fought with poisoned pigs. It is a detective story, a courtroom drama, a tragicomic farce, and a study of credulity and self-deception in the Age of Enlightenment.