To understand what happened at Rennes-le-Chateau between 1887 and 1891, we must step back and look at one of Europe's oldest dynasties, the Hapsburgs. The historical origin of the Hapsburg family goes back to the 10th century and the intrigues of Otto I, the first Hohenstauffen to become Emperor of Charlemagne's revived Holy Roman Empire. At that point the empire consisted of little more than a collection of Germanic states.
But in the mid 1880's, this was in the future and the Hapsburgs of Austria-Hungary were the most powerful ruling family in Europe. They enter the story of Rennes-le-Chateau with the arrival of the emissary, supposedly sent on behalf of the Comtesse de Chambord, the former Marie de Hapsburg-Lorraine, daughter of Leopold II, Archduke of Tuscany and the Piedmont of Italy, and widow of the Pretender, Henry V of France. Her titles and family connections are important, because it is likely that her supposed emissary was her youngest brother, Johann Salvator de Hapsburg-Lorraine. It is also likely that the Comtesse had nothing to do with it, beside providing a convenient cover story.
The Abbe Boudet sent his book to all the ruling families of Europe including the Hapsburgs, and given their history, the Hapsburg-Lorraines in particular. Descended from King Rene's grandson and his marriage to a Hapsburg princess in the late 15th century, the Hapsburg-Lorraines have often been called the Latinized Hapsburgs. Their long reign in northern Italy and Provence seems to have inspired these Hapsburgs with something lacking in their Austrian cousins.
However, by the middle of the 19th century, the Grand Dukes of Tuscany had fallen on hard times. Leopold II, Marie and Johann's father, backed his cousin the Emperor Franz Joseph in the struggles for Italian independence. After the Emperor's defeat at Soliferno in 1859, Leopold lost the Grand Duchy. It became part of Victor Emmnauel's new Kingdom of Italy. Leopold's eldest son Ferdinand renounced his claim to the throne, as did the middle brother Ludwig, who became a locksmith. Johann, the youngest, became the last hope of the family for greatness. It was a role that, subsequent history would suggest, he wore with some reluctance.
By 1887, Johann had already come into conflict with his cousin, the Emperor Franz Joseph, and ended any hope of advancement within the House of Hapsburg. Johann was a renegade archduke, a man with ambitions, a divine sense of authority, and no way to express his talents and abilities. He was friends with liberals, free thinkers, and his Christianity was in doubt, a serious question in Catholic Vienna. He was known to have joined several chivalric secret orders, such as the Knights of the Rosy-Cross and the Order of the Crescent founded by his ancestor, King Rene, in which he was the hereditary Grand Master. Given his background and interests, it would be surprising if Boudet's book had not reached him.
And so, in the spring of 1887, Archduke Johann paid a visit to the Abbe Sauniere under the guise of an emissary from his sister, Marie de Chambord, with a plausible cover story for looking for the Abbe Bigou's hidden genealogies. After Sauniere made his initial discovery, he did not try to contact the Comtesse, but he was in close contact with Archduke Johann. In the fall of 1887, bank accounts were opened on the same day in the same Austrian bank by both Johann and the Abbe Sauniere. Transactions between these two accounts would continue for years, even after Johann's disappearance. They would later form the basis of an accusation of spying leveled against Sauniere.
From the fall of 1887 to the summer of 1889, Johann seems to have been a frequent visitor to Rennes-le-Chateau. To the locals, he became the Stranger, and was remembered long afterward as Monsieur Guillame. During those visits, the three co-conspirators, Sauniere, Johann and Boudet, unraveled the mystery. It revealed a discovery so powerful that the Archduke Johann took it to his cousin, the Crown Prince Rudolph of Austria-Hungary, and thereby set off a train of events that led, inexorably, to the cataclysm of World War I.
By the spring of 1888, however, things were beginning to move. Johann renewed his contacts with various occult and chivalric orders, and, most important of all, he reconnected with his cousin, the Crown Prince Rudolph of Austria-Hungary. Rudolph, six years younger than Johann and Sauniere, was an unhappy, frustrated and idealistic young liberal heir to a vast, conservative and above all Catholic Empire. In 1888 Rudolph turned 30, and his father -- Franz Joseph had been on the throne for forty years -- showed no inclination toward sharing authority or including his son in the workings of government. This, in many ways, was the crowning frustration of a tortured and disturbed life. As a young boy, Rudolph's tutor, the Count Gondrecourt, bullied and abused him in a misguided attempt to toughen his nerve.
By the time he turned thirty in 1888, Rudolph's brilliance had begun to turn wild. Rudolph and Johann would be in continual contact for the next eight months until Rudolph's death. While the great secret, or even Rennes-le-Chateau, is never mentioned, there is evidence that Johann and Rudolph were planning some sort of coup. In the fall of 1888, Rudolph almost shot his father, Franz Joseph, "accidentally" on the hunting field. Rudolph was asked to leave for the day and the matter was hushed up. It was also the last time Franz Joseph saw his son alive. On January 30th 1889, Rudolph and his current mistress, Maria Vetsera, were found dead in the Imperial hunting lodge at Mayerling. Rumors leaked out that it was a suicide pact between the lovers. This was scandalous enough to quell any further investigation, and might just have been a smoke-screen. It is possible that Rudolph posed such a threat to the Empire that he was killed by the Imperial secret service.