Jimmy Rudge, chief of the infamous Gang "Jacks of Spades", kidnaps the Marquis d'Harcier and passes himself off as him. The impostor held large parties at his castle, in order to rob the invited guests. Charley Colms, a quirky private detective, gets hired to unmask the Marquis as a fake.

On his wedding night with Hilda Bergmann, the daughter of a swedish chemist, the young and rich american Willy Keanton is stabbed by an unidentified masked man. He kidnaps the young woman, who is found later on by the count Robert d'Arbel. The detective Jacques Miral, nicknamed « the pointed tower's ferret » is put in charge of the case. Soon, he discovers that an international bandit, Méphisto, seeks to seize the formula of a process against asphyxiating gas, invented by professor Bergmann. The struggle is tough between Miral and the uncatchable Méphisto. Monique Aubray, Keanton's secretary and engaged to the detective, the Nostradamus stallholders and the « mastiff of Bordeaux », and the famous novelist Fortuné Bidon are also involved in the case...

Historical film set during the Hundred Year War.