The Nature of the Evidence

The following night, the fourth night, she was to sleep in the spare room on the top floor, next to the servants, and Marston in his own room.

But Marston didn't sleep. He kept on wondering whether he would or would not go up to Pauline's room. That made him horribly restless, and instead of undressing and going to bed, he sat up on a chair with a book. He wasn't nervous; but he had a queer feeling that something was going to happen, and that he must be ready for it, and that he'd better be dressed.

It must have been soon after midnight when he heard the door-knob turning very slowly and softly.