A provincial chemist's daughter is to marry her cousin Gilbert.The day before he arrives, a stranger, who has just escaped from an insane asylum, claims he is the famous cousin.
France under Louis XIII. 1615. The Duke of Angoulême is at the head of a group of gentlemen who are conspiring to drive the Florentines out of court, of whom Concini is the all-powerful leader. A cadet from Gascony, Adhémar de Capestang, who was later to become the Capitan, left his native province to come and try his luck in Paris. Along the way, he saves a young girl, Gisèle d'Angoulême, from a masked individual who wanted to kidnap her.
At the English court, at the beginning of the 18th century, Lady Henriett fell in love with a farmer, Lionel, who, following a mistake, believes her to be his servant. By learning his quality, Lionel refuses to receive the one he already considered his fiancée. The queen herself will appease this lover's quarrel and, moreover, will free the peasants of his kingdom.
Mathilde Strangerson, the daughter of an eminent scientist, narrowly escapes being murdered in her own bedroom by an unknown assailant. As the room was locked from the inside, no one can understand how the attacker managed to enter or leave the room. Reporter and amateur sleuth Joseph Rouletabille arrives on the scene to protect Mathilde and resolve the mystery of the yellow room.
Nadia de Hock breaks her engagement to another man because of her love to the marquis d'Areghi. She lives a life of pleasure until Areghi reveals he is just an adventurer who has just killed his brother. He asks her to sign a letter so he can use her as his alibi. Some time later, Areghi is attacked by Nadia's guard dogs as he tries to see her. He dies as she forgives him for the pain he caused her.