“So what did you do?”

She said, “I was rattled, I guess, so I just automatically pushed the button which released the downstairs door catch. Then I ducked back into my own apartment, and hadn’t any more than got in when I heard the elevator stop at the seventh floor. I couldn’t understand that, because I knew these people couldn’t possibly have had time enough to get up to the seventh floor in the elevator. I waited, listening, and heard you two come down the corridor. As soon as the buzzer sounded in the other apartment, I opened the door to chase you away, but you were actually entering the apartment, so I had to make a quick excuse, that the sound of the buzzer had wakened me. Then I jerked the door shut. When the four people came up, I thought you were still in the apartment, and I had to see what was happening.”

“How long had you known him?” Mason asked.

She said sadly, “I loved him. I was the one that he wanted to marry when he left his wife. I don’t know how long this other romance had been going on. I became suspicious, and one time when I had an opportunity to go through his pockets, I found a key stamped, ‘Mandrake Arms Apartment, Number 702.’ Then I thought I knew, but I wanted to be sure. I found out who had Apartment 701 and made a proposition for a sublease that couldn’t be turned down.

“I waited and watched. This brunette walked down the corridor and used her key to open the apartment. I slipped out into the corridor and listened at the door. I heard him give her the same old line he’d given me so many times, and I hated him. I killed him—and I was caught.”

Mason turned to Stewart Linn and said, “There you are, young man. There’s your murderess, but you’ll probably never be able to get a jury to think it’s anything more than manslaughter.”

A much chastened Linn said, “Would you mind telling me how you figured this out, Mr. Mason?”

Mason said, “Clements’s key was missing. Obviously he must have had it when he entered the apartment. Therefore, the murderer must have taken it from his pocket. Why? So he or she could come back. And if what Don Ralston said was true, someone must have been in the apartment when he rang the bell from the street, someone who let him in by pressing the buzzer.

“What happened to that someone? I must have been walking down the corridor within a matter of seconds after Ralston had pressed the button on the street door. Yet I saw no one leaving the apartment. Obviously, then, the person who pressed the buzzer must have had a place to take refuge in a nearby apartment!