Mathilde Stangerson delays marrying Robert Darzac, as she wants to continue to aide her father, a scientist, in his experiments. Later, on the evening of her engagement announcement, Mathilde leaves her father in his laboratory at midnight, and goes to her adjoining yellow room. The professor, hearing gunshots and screaming, breaks Mathilde's locked door to find her bloodied, and the room in disarray, with papers of their studies stolen. How the assailant escaped the room, with a locked door and windows secured with iron shutters...