Once in the park, he stood, thus strangely attired, and with a ‘face of mystery’, as Mme Campan writes, peering out, from two separate locations, at the royal family and their train of attendants. Marie-Antoinette was deeply shocked and wanted to sack the porter the next day, but Mme Campan successfully intervened on his behalf.

What was His Eminence hoping for in the park? Was it to reveal himself in the confident expectation that her heart would melt when she saw him? Rohan was not that stupid: he had a paunch, he was no longer young, and the ludicrous disguise would hardly have advanced his cause. The only possible explanation is that he desperately wanted to see the Queen, and that was why he had gone there.

But whatever the case, there is no doubt that he was driven by the desire, verging on compulsion, to diffuse the Queen’s anger and win her favour. This is the second such idée fixe in the story, according to Carlyle, the first being Boehmer’s with his necklace. And when two such obsessions come together, a force comes into being that could destroy a nation. All that is needed is for them to combine with a third.

And that was how Jeanne de la Motte found Rohan when she met him at Saverne, the Bishop of Strasbourg’s country seat. The old manor house had burnt down in 1779, but Rohan had rebuilt it in fashionable pomp and splendour and fitted it out, again in the taste of the time, with collections of natural history and art, and splendid libraries. The number of his guests had not diminished since his days in Vienna. He lived like a prince. They came in such numbers, from all over Germany and France, and even from the Court at Versailles, that often there was no room for them all in the mansion, despite its seven hundred awaiting beds. “There was no noblewoman of such good family that she did not dream of Saverne,” wrote a contemporary. “The hunts were especially magnificent.” Six hundred peasants drove the game into the gentlemen’s guns, with the women following on horseback or in carriages. At one o’clock the entire party assembled for luncheon, in a marquee erected in some picturesque spot on the banks of a stream. So that the pleasure should be shared by everyone, there were even tables waiting on the lawn for the peasantry: it was Rohan’s wish that every one of them should have a pound of meat, two pounds of bread and half a bottle of wine. At Saverne they certainly enjoyed to the full what Talleyrand calls ‘the sweetness of life’.

It was to this fairytale castle that Jeanne de la Motte, dissatisfied and inwardly eaten up as ever, came with her husband. Her great patron Mme de Boulainvilliers introduced her to the Cardinal and commended her to his favour. She told him her story while he listened in rapt silence—which is hardly surprising, given the details that might have come from a novel. The unvarnished realities of a defenceless life, the bitter taste of poverty, would have been particularly fascinating to a man whose own days had been passed in the most cushioned elevation, and to whom the woes of ordinary life were comprehensible only as some sort of exotic and compelling tale. And Jeanne knew supremely well how to present her tale with steadily mounting effect. On a number of occasions her later writings reveal—it is quite noticeable—that she had practised extensively and this was the form she finally evolved. One can imagine her using her spare moments to rehearse it over and over again, honing it down to one particular version with its ever-increasing drama.

So Rohan listened, and believed everything. He usually believed what people told him. Two days before his arrest, Cagliostro persuaded him that he would be dining with Henry IV, though he would not actually see his illustrious guest. If one were writing a play on the subject one would have to ignore everything else and focus on this trait alone, because in the entire drama of the necklace it is the most significant. He showed the most extraordinary, indeed unbelievable, gullibility. The most problematic aspect of the whole affair, Funck-Brentano tells us, was the degree of credulity we are required to attribute to him. It is the most improbable feature in the whole improbable story—but it is undeniable.

The most obvious explanation for it, other than some character trait of unknown origin, can once again lie only in his social position. How would anyone born into the purple, and destined to become a cardinal, get to know people, the circumstances of their lives—and the sheer nastiness circumstances can provoke in them? Any other nobleman, busying himself with affairs of state or military matters, would have rapidly discovered what people are really like. But Rohan was a man of the Church, and he took no interest in his diocese. People showed him only their better side and revealed only the noblest of their motives to him.