'Possibly, mademoiselle. Nevertheless, he was an old friend of M. Déroulard's. There may be things he can tell me - things of the past - old grudges - old love-affairs.'

The girl flushed and bit her lip. 'As you please - but - but - I feel sure now that I have been mistaken. It was good of you to accede to my demand, but I was upset - almost distraught at the time. I see now that there is no mystery to solve. Leave it, I beg of you, monsieur.'

I eyed her closely.

'Mademoiselle,' I said, 'it is sometimes difficult for a dog to find a scent, but once he has found it, nothing on earth will make him leave it! That is if he is a good dog! And I, mademoiselle, I, Hercule Poirot, am a very good dog.'

Without a word she turned away. A few minutes later she returned with the address written on a sheet of paper. I left the house. François was waiting for me outside. He looked at me anxiously.

'There is no news, monsieur?'

'None as yet, my friend.'

'Ah! Pauvre Monsieur Déroulard!' he sighed. 'I too was of his way of thinking. I do not care for priests. Not that I would say so in the house. The women are all devout - a good thing perhaps. Madame est très pieuse - et Mademoiselle Virginie aussi.'

Mademoiselle Virginie? Was she 'très pieuse?' Thinking of the tear-stained passionate face I had seen that first day, I wondered.

Having obtained the address of M. de Saint Alard, I wasted no time. I arrived in the neighbourhood of his château in the Ardennes but it was some days before I could find a pretext for gaining admission to the house. In the end I did - how do you think - as a plumber, mon ami! It was the affair of a moment to arrange a neat little gas leak in his bedroom. I departed for my tools, and took care to return with them at an hour when I knew I should have the field pretty well to myself. What I was searching for, I hardly knew. The one thing needful, I could not believe there was any chance of finding. He would never have run the risk of keeping it.

Still when I found a little cupboard above the washstand locked, I could not resist the temptation of seeing what was inside it. The lock was quite a simple one to pick. The door swung open. It was full of old bottles. I took them up one by one with a trembling hand. Suddenly, I uttered a cry. Figure to yourself, my friend, I held in my hand a little phial with an English chemist's label. On it were the words: 'Trinitrine Tablets. One to be taken when required. Mr John Wilson.'

I controlled my emotion, closed the little cupboard, slipped the bottle into my pocket, and continued to repair the gas leak! One must be methodical. Then I left the château, and took train for my own country as soon as possible. I arrived in Brussels late that night. I was writing out a report for the préfet in the morning, when a note was brought to me. It was from old Madame Déroulard, and it summoned me to the house in the Avenue Louise without delay.