Le Secret du Masque de fer (The Secret of the Iron mask) is a historical essay by French novelist Marcel Pagnol, who identified the famous prisoner in the iron mask as the twin brother of Louis XIV, born after him and imprisoned for life in 1669 for having conspired against the King. The essay was published for the first time in 1965 under the title Le Masque de fer (The Iron Mask), and updated in 1973, completed in particular with research on James de la Cloche, identified as the twin bearing this name in his youth.
Raised by the midwife Lady Perronette, the twin was taken to the island of Jersey at the age of six, where he was brought up by Marguerite Carteret, daughter of the island's noblest family. Having converted to Catholicism in 1667, he entered the Jesuit novitiate in Rome in April 1668. He then returned to London at the end of 1668 where he is thought to have conspired against Louis XIV alongside a certain Roux de Marcilly, who was denounced and executed in June 1669.
The accomplice of Roux, passing for his valet, Martin, and whom Marcel Pagnol in turn identified as the twin brother of Louis XIV, was also arrested and taken to Calais in July 1669, then brought to Pignerol prison under the custody of the governor Saint-Mars, where he remained until 1681. He was given the name of "Eustache Dauger", designating him as a simple valet. Still under the custody of Saint Mars, he was transferred to the fort of Exilles then to the island of Sainte-Marguerite in 1687, and finally to the Bastille in 1698, where he died in 1703 after 34 years in captivity.
Marcel Pagnol also provides information intended to demonstrate that the prisoner cannot be identified as a valet or as Count Matthioli. He thus contradicts other historians' theories which, in his view, resulted from false information deliberately disseminated by the authorities of the period who, foreseeing later examination of the correspondence, supposedly laid false trails to try to conceal the prisoner's real identity.