There was but one door leading into Mr. Deacon's rooms from the open corridor of the building, and this was immediately opposite the large street door. When one entered from the street one ascended three or four broad marble steps, pushed open one of a pair of glazed swing doors and found oneself facing the door by which Mr. Deacon entered and left his quarters. There had originally been other doors into the corridor from some of the rooms, but those Mr. Deacon had had blocked up, so making the flat entirely self-contained. Just by the glazed swing doors which I have spoken of, and in full view of the old gentleman's door, the hall-porter's box stood. It was glazed on all sides, and the porter sat so that Mr. Deacon's door was always before his eyes, and, so long as he was there, it was very unlikely that anybody or anything could leave or enter by that door unobserved by him. It is important to remember this, in view of what happened on the occasion I am writing of. There was one other exterior door to Mr. Deacon's flat, and one only. It gave upon the back spiral staircase, and was usually kept locked. This staircase had no outlet to the corridors, but merely extended from the housekeeper's rooms at the top of the building to the basement. It was little used, and then only by servants, for it gave access only to the rooms on its own side. There was no way from this staircase to the outer street except through the private rooms of the tenants, or through those of the housekeeper. That Wednesday morning things had happened precisely in the ordinary way. Mr. Deacon had risen and breakfasted as usual. He was alone, with his newspaper and his morning letters, when his breakfast was taken in and when it was removed. He had remained in his rooms till between twelve and one o'clock. Goods had arrived for him (this was an almost daily occurrence), and one or two ordinary visitors had called and gone away again. It was Mr. Deacon's habit to lunch at his club, and at about a quarter to one, or thereabout, he had come out, locked his door, and leaving his usual message that he should be at the club for an hour or two, in case anybody called, he had left the building. At about one, however, he had returned hurriedly, having forgotten some letters. "I didn't give you any letters for the post, did I, Beard, before I went out?" he asked the porter. And the porter replied that he had not. Mr. Deacon thereupon crossed the corridor, entered his door, and shut it behind him.