The journalist Fandor befriends the King of Hesse-Weimar during one of his Majesty's incognito pleasure trips to Paris, but when the King's mistress is found dead, having fallen from a third-floor window, Inspector Juve is called upon to investigate. When the indomitable detective discovers Lady Beltham, the lover of Fantômas, in disguise among the royal court of Hesse-Weimar, Juve suspects it is none other than the Lord of Terror who is behind the plot against the King.