"Do you mean we are to slip off again at the last minute?"

"No, Hastings. Our enemies will be satisfied with nothing less than a bonafide departure."

"But the train doesn't stop until Calais?"

"It will stop if it is paid to do so."

"Oh, come now, Poirot-surely you can't pay an express to stop-they'd refuse."

"My dear friend, have you never remarked the little handle-the signale d'arret- penalty for improper use, 100 francs, I think?"

"Oh! you are going to pull that?"

"Or rather a friend of mine, Pierre Combeau, will do so. Then, while he is arguing with the guard, and making a big scene, and all the train is agog with interest, you and I will fade quietly away."

We duly carried out Poirot's plan. Pierre Combeau, an old crony of Poirot's, and who evidently knew my little friend's methods pretty well, fell in with the arrangements.

The communication cord was pulled just as we got to the outskirts of Paris. Combeau "made a scene" in the most approved French fashion and Poirot and I were able to leave the train without any one being interested in our departure. Our first proceeding was to make a considerable change in our appearance. Poirot had brought the materials for this with him in a small case. Two loafers in dirty blue blouses were the result.

We had dinner in an obscure hostelry, and started back to Paris afterwards.

It was close on eleven o'clock when we found ourselves once more in the neighbourhood of Madame Olivier's villa. We looked up and down the road before slipping into the alleyway. The whole place appeared to be perfectly deserted. One thing we could be quite certain of, no one was following us.

"I do not expect them to be here yet," whispered Poirot to me. "Possibly they may not come until tomorrow night, but they know perfectly well that there are only two nights on which the radium will be there."

Very cautiously we turned the key in the garden door.

It opened noiselessly and we stepped into the garden.

And then, with complete unexpectedness, the blow fell. In a minute we were surrounded, gagged and bound. At least ten men must have been waiting for us.

Resistance was useless. Like two helpless bundles we were lifted up and carried along. To my intense astonishment, they took us towards the house and not away from it. With a key they opened the door into the laboratory and carried us into it. One of the men stooped down before the big safe. The door of it swung open. I felt an unpleasant sensation down my spine. Were they going to bundle us into it, and leave us there to asphyxiate slowly?

However, to my amazement, I saw that from the inside of the safe steps led down beneath the floor. We were thrust down this narrow way and eventually came out into a big subterranean chamber. A woman stood there, tall and imposing, with a black velvet mask covering her face. She was clearly in command of the situation by her gestures of authority. The men slung us down on the floor and left us-alone with the mysterious creature in the mask. I had no doubt who she was.

This was the unknown Frenchwoman-Number Three of the Big Four.

She knelt down beside us and removed the gags, but left us bound, then rising and facing us, with a sudden swift gesture she removed her mask.

It was Madame Olivier!

"M. Poirot," she said, in a low mocking tone. "The great, the wonderful, the unique M. Poirot. I sent a warning to you yesterday morning. You chose to disregard it-you thought you could pit your wits against US. And now, you are here!"

There was a cold malignity about her that froze me to the marrow. It was so at variance with the burning fire of her eyes. She was mad-mad-with the madness of genius!

Poirot said nothing. His jaw had dropped, and he was staring at her.

"Well," she said softly, "this is the end. WE cannot permit our plans to be interfered with. Have you any last request to make?"

Never before, or since, have I felt so near death.

Poirot was magnificent. He neither flinched nor paled, just stared at her with unabated interest. 

"Your psychology interests me enormously, madame," he said quietly. "It is a pity that I have so short a time to devote to studying it. Yes, I have a request to make. A condemned man is always allowed a last smoke, I believe. I have my cigarette case on me. If you would permit-" He looked down at his bonds.

"Ah, yes!" she laughed. "You would like me to untie your hands, would you not? You are clever, M. Hercule Poirot, I know that. I shall not untie your hands-but I will find you a cigarette."

She knelt down by him, extracted his cigarette case, took out a cigarette, and placed it between his lips.

"And now a match," she said, rising.

"It is not necessary, madame." Something in his voice startled me. She, too, was arrested.

"Do not move, I pray of you, madame. You will regret it if you do. Are you acquainted at all with the properties of cuare? The South American Indians use it as an arrow poison. A scratch with it means death.

Some tribes use a little blow-pipe-I, too, have a little blow-pipe constructed so as to look exactly like a cigarette. I have only to blow… Ah! you start. Do not move, madame. The mechanism of this cigarette is most ingenious. One blows-and a tiny dart resembling a fishbone flies through the air-to find its mark. You do not wish to die, madame. Therefore, I beg of you, release my friend Hastings from his bonds. I cannot use my hands, but I can turn my head-so-you are still covered, madame. Make no mistake, I beg of you."

Slowly, with shaking hands, and rage and hate convulsing her face, she bent down and did his bidding. I was free. Poirot's voice gave me instructions.

"Your bonds will now do for the lady, Hastings. That is right. Is she securely fastened? Then release me, I pray of you. It is a fortunate circumstance she sent away her henchmen. With a little luck we may hope to find the way out unobstructed."

In another minute, Poirot stood by my side. He bowed to the lady.

"Hercule Poirot is not killed so easily, madame. I wish you goodnight."

The gag prevented her from replying, but the murderous gleam in her eyes frightened me. I hoped devoutly that we should never fall into her power again.

Three minutes later we were outside the villa, and hurriedly traversing the garden. The road outside was deserted, and we were soon clear of the neighbourhood.

Then Poirot broke out.

"I deserve all that that woman said to me. I am a triple imbecile, a miserable animal, thirty-six times an idiot. I was proud of myself for not falling into their trap. And it was not even meant as a trap-except exactly in the way in which I fell into it. They knew I would see through it-they counted on my seeing through it. This explains all-the ease with which they surrendered Halliday-everything. Madame Olivier was the ruling spirit-Vera Rossakoff only her lieutenant.

Madame needed Halliday's ideas-she herself had the necessary genius to supply the gaps that perplexed him.

Yes, Hastings, we know now who Number Three is-the woman who is probably the greatest scientist in the world! Think of it. The brain of the East, the science of the West-and two others whose identities we do not yet know. But we must find out. To-morrow we will return to London and set about it."

"You are not going to denounce Madame Olivier to the police?"

"I should not be believed. That woman is one of the idols of France. And we can prove nothing. We are lucky if she does not denounce us."