It was a long time before there was any response. Duvall had almost begun to despair of getting one, when he heard the clicking of the electric latch, and found that he could turn the knob and enter the hallway. He had barely done so, when at big, burly-looking man, who might have been a bartender, or a head waiter, appeared in the door of one of the ground floor apartments, clad only in his night clothes. "Well--whatcha want?" he growled. Duvall stepped up to him quickly, and spoke in a pleasant voice. "I'm mighty sorry," he said. "I rang your bell by mistake. Pardon me." The man glared at him, suspicion blazing from his eyes. "That's an old one," he retorted. "How do I know you ain't a burglar?" "Do I look like one?" Duvall asked. The man ignored this question. "Rang my bell by mistake, did you? Who do you want to see?" "I have some business with a lady on the fourth floor." He went closer to the man, and lowered his voice. "I'm a detective, my friend," he whispered confidentially. "I'm here on a very important case." The big man's eyes widened. "Th' hell you are!" he exclaimed. "Central office?" "No. Private." "H--m." The man nodded slowly. "All right. But I guess I'll keep my eye on you, just the same." He leaned against the door jamb and watched Duvall as he ascended the stairs. The detective reached the fourth floor at top speed. He was panting, when he arrived opposite the door of the apartment he sought. Once there, he paused for a moment, listening intently. Not a sound came from the interior of the flat. The problem of obtaining access to the place now confronted him. The door was of oak of stout construction. He doubted his ability to break it in, nor did he wish to attempt to do so, if it could be avoided. Breaking into private apartments, without a warrant, was a serious matter. There was a chance that this might not be the right place, after all. He hesitated. Yet Grace might be within, in danger, perhaps, of her life. It was imperative that he should find out the truth at once. Stepping up to the door, he knocked sharply upon it, then waited for a reply. He scarcely expected one, but felt that he should at least give the persons within a chance. A long silence ensued. Duvall was about to rap again, when, to his amazement, the door slowly and noiselessly swung inward, as though impelled by some unseen hand. The room beyond was shrouded in darkness. Duvall could see no one. Whoever had opened the door must now be concealed behind it. No one either greeted or challenged him. The door swung three-quarters open, and stood still. Not a sound was to be heard. The room was as silent as a tomb. Duvall stood on the threshold for a few seconds, listening intently. He was greatly astonished by what had occurred. Why had the door been so silently opened? Was someone waiting within, ready to attack him the moment he made a step forward?