“There is another possibility. She may have forgotten to bolt the door into the passage when she went to bed, and have got up later, towards morning, and bolted it then.”

“Poirot, is that seriously your opinion?”

“No, I do not say it is so, but it might be. Now, to turn to another feature, what do you make of the scrap of conversation you overheard between Mrs. Cavendish and her mother-in-law?”

“I had forgotten that,” I said thoughtfully. “That is as enigmatical as ever. It seems incredible that a woman like Mrs. Cavendish, proud and reticent to the last degree, should interfere so violently in what was certainly not her affair.”

“Precisely. It was an astonishing thing for a woman of her breeding to do.”