And it was in this sombre and desolate retreat that we saw the white walls of the pavilion as we approached. A queer-looking building without a window visible on the side by which we neared it. A little door alone marked the entrance to it. It might have passed for a tomb, a vast mausoleum in the midst of a thick forest. As we came nearer, we were able to make out its disposition. The building obtained all the light it needed from the south, that is to say, from the open country. The little door closed on the park. Monsieur and Mademoiselle Stangerson must have found it an ideal seclusion for their work and their dreams. [Illustration: A ground plan of the pavilion, a square building, situated such that its north and east sides connect with a wall that encloses the surrounding park. The building contains four rooms, labeled “1” through “4”. The front door is in the southwest corner and leads into room 3. A door in the north side of room 3 leads into room 4, a narrow room by a staircase labeled “5”. Room 1 is in the northwest corner and contains a bed in one corner. Room 2 fully occupies the right half of the building. It has doors connecting to rooms 1 and 3, and windows on the north and east walls. Against its south wall is a rectangle labeled “6”. There is a trail of footsteps marked on the plan, starting just outside a window in room 3 and leading off into the park.] Here is the ground plan of the pavilion. It had a ground-floor which was reached by a few steps, and above it was an attic, with which we need not concern ourselves. The plan of the ground-floor only, sketched roughly, is what I here submit to the reader. 1. The Yellow Room, with its one window and its one door opening into the laboratory. 2. Laboratory, with its two large, barred windows and its doors, one serving for the vestibule, the other for The Yellow Room. 3. Vestibule, with its unbarred window and door opening into the park. 4. Lavatory. 5. Stairs leading to the attic. 6. Large and the only chimney in the pavilion, serving for the experiments of the laboratory. The plan was drawn by Rouletabille, and I assured myself that there was not a line in it that was wanting to help to the solution of the problem then set before the police. With the lines of this plan and the description of its parts before them, my readers will know as much as Rouletabille knew when he entered the pavilion for the first time. With him they may now ask: How did the murderer escape from The Yellow Room? Before mounting the three steps leading up to the door of the pavilion, Rouletabille stopped and asked Monsieur Darzac point blank:— “What was the motive for the crime?”