'Then I saw my path clear. He was my son. I had given him life. I was responsible for him. He had killed one woman's body, now he would kill another's soul! I went to Mr Wilson's room, and took the bottle of tablets. He had once said laughingly that there were enough in it to kill a man! I went into the study and opened the big box of chocolates that always stood on the table. I opened a new box by mistake. The other was on the table also. There was just one chocolate left in it. That simplified things. No one ate chocolates except my son and Virginie. I would keep her with me that night. All went as I had planned -'

She paused, closing her eyes a minute then opened them again.

'M. Poirot, I am in your hands. They tell me I have not many days to live. I am willing to answer for my action before the good God. Must I answer for it on earth also?'

I hesitated. 'But the empty bottle, madame,' I said to gain time. 'How came that into M. de Saint Alard's possession?'

'When he came to say goodbye to me, monsieur, I slipped it into his pocket. I did not know how to get rid of it. I am so infirm that I cannot move about much without help, and finding it empty in my rooms might have caused suspicion. You understand, monsieur -' she drew herself up to her full height - 'it was with no idea of casting suspicion on M. de Saint Alard! I never dreamed of such a thing. I thought his valet would find an empty bottle and throw it away without question.'

I bowed my head. 'I comprehend, madame,' I said.

'And your decision, monsieur?'

Her voice was firm and unfaltering, her head held as high as ever. I rose to my feet.

'Madame,' I said, 'I have the honour to wish you good day. I have made my investigations - and failed! The matter is closed.'

He was silent for a moment, then said quietly: 'She died just a week later. Mademoiselle Virginie passed through her novitiate, and duly took the veil. That, my friend, is the story. I must admit that I do not make a fine figure in it.'

'But that was hardly a failure,' I expostulated. 'What else could you have thought under the circumstances?'

'Ah, sacré, mon ami,' cried Poirot, becoming suddenly animated. 'Is it that you do not see? But I was thirty-six times an idiot! My grey cells, they functioned not at all. The whole time I had the true clue in my hands.'

'What clue?'

'The chocolate box! Do you not see? Would anyone in possession of their full eyesight make such a mistake? I knew Madame Déroulard had cataract - the atropine drops told me that. There was only one person in the household whose eyesight was such that she could not see which lid to replace. It was the chocolate box that started me on the track, and yet up to the end I failed consistently to perceive its real significance!