I stared at the old lady, feeling an increased respect for her mental powers. Her keen wits had seen what we had failed to perceive. It was an odd thing—a very odd thing.
“If only,” I said, “the letter hadn’t been dated—”
Miss Marple nodded her head.
“Exactly,” she said. “If it hadn’t been dated!”
I cast my mind back, trying to recall that sheet of notepaper and the blurred scrawl, and at the top that neatly printed 6:20. Surely these figures were on a different scale to the rest of the letter.
I gave a gasp.
“Supposing,” I said, “it wasn’t dated. Supposing that round about 6:30 Colonel Protheroe got impatient and sat down to say he couldn’t wait any longer. And as he was sitting there writing, someone came in through the window—”
“Or through the door,” suggested Griselda.
“He’d hear the door and look up.”
“Colonel Protheroe was rather deaf, you remember,” said Miss Marple.
“Yes, that’s true. He wouldn’t hear it. Whichever way the murderer came, he stole up behind the colonel and shot him. Then he saw the note and the clock and the idea came to him. He put 6:20 at the top of the letter and he altered the clock to 6:22. It was a clever idea. It gave him, or so he would think, a perfect alibi.”