Several times did he ring, and although he heard the shrill vibration of the bell echo through the house, no one appeared in answer to its imperative summons. Thinking he might have made a mistake, the constable stepped back into the garden. But he was right. This was the villa out of which the young man had issued, for there burned the guiding light on the first floor. Mulligan felt puzzled by the inexplicable silence and rang the bell again. Indeed he pressed his great thumb on the ivory button for nearly one minute. The bell shrilled continuously and imperiously. Still no one came. Mulligan scratched his head and considered. "Something's wrong," thought he. "If I'd the key I'd enter and see if the lady is ill. Queer, the bell don't waken the servants. Augh! The lazy beasts." It occurred to him that in his hand he held the key dropped by the young gentleman. Almost without thinking he fumbled for the hole and slipped in the key. To his surprise it turned under his involuntary pressure, and the door swung open noiselessly. Again the constable scratched his head. Things--so he assured himself--were becoming mysterious, and he scented an adventure. It was strange that this key should open the door. "Unless this is his home, and he's running away for some devilment. Maybe the lady isn't his sister; perhaps his wife or his sweetheart. Augh! But she'd not let him go at this hour. Catch her." However he might argue, it was foolish to stand before an open door without doing something. The inspector would be round soon, and might--probably would--demand an explanation. Now that he had got this far, Mulligan naturally decided to see the adventure through. As yet he had no suspicion that anything was wrong, though he certainly thought the whole affair mysterious. Walking into the dark hall, at the end of which, by the light of his lantern, he saw the glimmer of a marble staircase, he called gently up into the blackness. "Is there any one there?" demanded Mulligan. "If so, come down, for I'm in want of an explanation." He paused and listened. There came no reply. The dense silence held the house. Not even a clock ticked. Mulligan suppressed his breath and listened with all his ears. No sound filled them save the drumming of his heart. Again he ran into the garden and again assured himself that the light was burning overhead. He began to conclude that the position called for the intervention of the law. Assuming an official air, he tramped up the stairs, flashing the light right and left as he ascended. He did not know the position of the room, save that it was in the front of the house. But thus indicated, he thought there would be little difficulty in finding it and solving the mystery.