He drugs the wine which is drunk by the detective, and as the latter is examining Larson's hair he notices a bad wound in the inspector's hand and then falls unconscious. Larson, who was none other than Roussel, sends Mathilda another note and fearing the surveillance of Rouletabille, he invites the detective to supper in his room. He is also surprised to learn that Larson's hair is the color of that which he had found in Mathilda's boudoir. Previously the detective had caught a glimpse of a bearded man and has come to the conclusion that he must be in the house. At midnight the detective hears sounds coming from the yellow room, and stations Larson and the professor at both ends of the gallery, but no one is found.

Then Mathilda receives another letter from Roussel, which makes her change her room. Professor Stangerson places the solving of the mystery in the hands of Rouletabille, a noted detective, and inspector Larson, They are given adjoining rooms in the Stangerson castle, and in his work of unraveling the enigma, Rouletabille finds under the bed of Mathilda a hair and a bloody handkerchief. Upon investigation the father finds that his daughter is lying unconscious upon the floor with a deep gash in her head, but no trace of her assailant could be found for the doors and windows were all locked from the inside. She does and no sooner had the household retired when a shot comes from the room of Mathilda. That evening as she joins her father he notices that she is unusually pale and advises her to retire early. In order to stop her, Roussel chokes her into unconsciousness and leaves her for dead.

But Mathilda was very much afraid and screams. Mathilda, who had been out walking with her father and fiancé, returned and feeling tired goes direct to her room, where she comes face to face with Roussel, who cautions her not to utter a word of alarm. Mathilda was too frightened to answer, so Roussel in a jealous rage goes to the residence of Mathilda and hides in the yellow room occupied by her. By a subterfuge he manages to get a note to her telling her that he still loves her and begging her to flee with him to America. Roussel, however, was still alive and soon learns of the engagement of Mathilda. But she soon dismisses the horror upon the thought that Roussel must have been dead (she never having heard from or of him) and at last consents to the announcement of her betrothal to Darzac. A reminiscence of her act ten years ago, recalling how she had secretly married Jean Roussel, flashes through the mind of Mathilda, daughter of Professor Stangerson, when her father asks her to become the wife of Robert Darzac, and how their wedding certificate could not be filed as her husband was imprisoned for passing counterfeit money.

The mystery of the yellow room