"That settles it," I breathed. "These were no ordinary thieves. This one stone was all they wanted."

"But how did the fellow get in?"

"Through this door."

"But it's always locked."

I shook my head. "It's not locked now. See." I pulled it open as I spoke. As I did so something fluttered to the ground. I picked it up. It was a piece of silk, and the embroidery was unmistakable. It had been tom from a Chinaman's robe.

"In his haste it caught in the door," I explained. "Come, hurry. He cannot have gone far as yet." But in vain we hunted and searched. In the pitch darkness of the night, the thief had found it easy to make his getaway. We returned reluctantly, and Lord Yardly sent off one of the footmen posthaste to fetch the police. Lady Yardly, aptly ministered to by Poirot, who is as good as a woman in these matters, was sufficiently recovered to be able to tell her story.

"I was just going to turn on the other light," she said, "when a man sprang on me from behind. He tore my necklace from my neck with such force that I fell headlong to the floor. As I fell I saw him disappearing through the side door. Then I realized by the pig-tail and the embroidered robe that he was a Chinaman."

She stopped with a shudder.

The butler reappeared. He spoke in a low voice to Lord Yardly.

"A gentleman from Mr Hoffberg's, m'lord. He says you expect him."

"Good heavens!" cried the distracted nobleman. "I must see him, I suppose. No, not here, Mullings, in the library."

I drew Poirot aside.

"Look here, my dear fellow, hadn't we better get back to London?"

"You think so, Hastings? Why?"

"Well -" I coughed delicately - "things haven't gone very well, have they? I mean, you tell Lord Yardly to place himself in your hands and all will be well - and then the diamond vanishes from under your very nose!"

"True," said Poirot, rather crestfallen. "It was not one of my most striking triumphs." This way of describing events almost caused me to smile, but I stuck to my guns.

"So, having - pardon the expression - rather made a mess of things, don't you think it would be more graceful to leave immediately?"

"And the dinner, the without doubt excellent dinner, that the chef of Lord Yardly has prepared?"

"Oh, what's dinner!" I said impatiently.

Poirot held up his hands in horror.

"Mon Dieu! It is that in this country you treat the affairs gastronomic with a criminal indifference."

"There's another reason why we should get back to London as soon as possible," I continued.

"What is that, my friend?"

"The other diamond," I said, lowering my voice. "Miss Marvell's."

"Eh bien, what of it?"

"Don't you see?" His unusual obtuseness annoyed me. What had happened to his usually keen wits? "They've got one, now they'll go for the other."

"Tiens!" cried Poirot, stepping back a pace and regarding me with admiration. "But your brain marches to a marvel, by friend! Figure to yourself that for the moment I had not thought of that! But there is plenty of time. The full of the moon, it is not until Friday." I shook my head dubiously. The full of the moon theory left me entirely cold. I had my way with Poirot, however, and we departed immediately, leaving behind us a note of explanation and apology for Lord Yardly.