Dans le Paris débridé des année folles, alors que le gouvernement est aux abois, on ne parle que d'un mystérieux malfaiteur, tout-puissant, «le Grand X». Un riche banquier et homme politique, Milion-Lauenbourg, est le père d'une très jolie fille, Sylvie, amoureuse d'un député adversaire de son père, Claude Corbières. Celui-ci dirige la ligue anti parlementaire. Milion-Lauenbourg dirige ses affaires grâce à l'aide inestimable de M. Barnabé, aux allures de petit bureaucrate radin, mais qui en fait connaît tout de la banque. Il a recours aux services de Dumont, chef de la Sureté, prototype du policier avide de pouvoir. D'autres comparses gravitent autour de ces personnages, hommes politiques, truands, parents...

Gaston Leroux:Gaston Louis Alfred Leroux was a French journalist and author of detective fiction.In the English-speaking world, he is best known for writing the novel The Phantom of the Opera (Le Fantôme de l'Opéra, 1910), which has been made into several film and stage productions of the same name, such as the 1925 film starring Lon Chaney, and Andrew Lloyd Webber's 1986 musical. It was also the basis of the 1990 novel Phantom by Susan Kay.Leroux went to school in Normandy and studied law in Paris, graduating in 1889. He inherited millions of francs and lived wildly until he nearly reached bankruptcy. Then in 1890, he began working as a court reporter and theater critic for L'Écho de Paris. His most important journalism came when he began working as an international correspondent for the Paris newspaper Le Matin. In 1905 he was present at and covered the Russian Revolution. Another case he was present at involved the investigation and deep coverage of an opera house in Paris, later to become a ballet house. The basement consisted of a cell that held prisoners in the Paris Commune, which were the rulers of Paris through much of the Franco-Prussian war.