Feathergill, an assistant District Attorney, was waiting in the main hall for his Chief’s arrival.

“It’s too bad, sir,” he lamented. “A rotten show all round. And just at this time! …” He shrugged his shoulders discouragingly.

“It may collapse quickly,” said Markham, shaking the other’s hand. “How are things going? Sergeant Heath phoned me right after you called, and said that, at first glance, the case looked a bit stubborn.”

“Stubborn?” repeated Feathergill lugubriously. “It’s downright impervious. Heath is spinning round like a turbine. He was called off the Boyle case, by the way, to devote his talents to this new shocker. Inspector Moran arrived ten minutes ago, and gave him the official imprimatur.”

“Well, Heath’s a good man,” declared Markham. “We’ll work it out. … Which is the apartment?”

Feathergill led the way to a door at the rear of the main hall.

“Here you are, sir,” he announced. “I’ll be running along now. I need sleep. Good luck!” And he was gone.

It will be necessary to give a brief description of the house and its interior arrangement, for the somewhat peculiar structure of the building played a vital part in the seemingly insoluble problem posed by the murder.

The house, which was a four-story stone structure originally built as a residence, had been remodelled, both inside and outside, to meet the requirements of an exclusive individual apartment dwelling. There were, I believe, three or four separate suites on each floor; but the quarters up-stairs need not concern us. The main floor was the scene of the crime, and here there were three apartments and a dentist’s office.

The main entrance to the building was directly on the street, and extending straight back from the front door was a wide hallway. Directly at the rear of this hallway, and facing the entrance, was the door to the Odell apartment, which bore the numeral “3.” About half-way down the front hall, on the right-hand side, was the stairway leading to the floors above; and directly beyond the stairway, also on the right, was a small reception-room with a wide archway instead of a door. Directly opposite to the stairway, in a small recess, stood the telephone switchboard. There was no elevator in the house.

Another important feature of this ground-floor plan was a small passageway at the rear of the main hall and at right angles to it, which led past the front walls of the Odell apartment to a door opening on a court at the west side of the building. This court was connected with the street by an alley four feet wide.

In the accompanying diagram this arrangement of the ground floor can be easily visualized, and I suggest that the reader fix it in his mind; for I doubt if ever before so simple and obvious an architectural design played such an important part in a criminal mystery. By its very simplicity and almost conventional familiarity—indeed, by its total lack of any puzzling complications—it proved so baffling to the investigators that the case threatened, for many days, to remain forever insoluble.

As Markham entered the Odell apartment that morning Sergeant Ernest Heath came forward at once and extended his hand. A look of relief passed over his broad, pugnacious features; and it was obvious that the animosity and rivalry which always exist between the Detective Division and the District Attorney’s office during the investigation of any criminal case had no place in his attitude on this occasion.