Belphégor deals with a series of mysterious appearances by a masked-and-robed figure in the Louvre; a security guard is murdered, and a later police trap is foiled when the phantom—“Belphégor” (the name of a legendary demon)—uses knock-out gas. Journalist Jacques Bellegarde of “Le Petit Parisien” (the real-life newspaper which published the original story in serial installments), investigates, and eventually discovers famous detective Chantecoq and his vivacious daughter Colette are also on the case.