“I suppose so, yes.”

“Anyone with a minimum of intelligence would appreciate the cash value of the plans?”

“Yes, but M. Poirot—” Lord Mayfield was looking rather uncomfortable.

Poirot held up a hand.

“I do what you call explore all the avenues.”

Suddenly he rose again, stepped nimbly out of the window and with a flashlight examined the edge of the grass at the farther side of the terrace.

The two men watched him.

He came in again, sat down and said:

“Tell me, Lord Mayfield, this malefactor, this skulker in the shadows, you do not have him pursued?”

Lord Mayfield shrugged his shoulders.

“At the bottom of the garden he could make his way out to a main road. If he had a car waiting there, he would soon be out of reach—”

“But there are the police—the A.A. scouts—”

Sir George interrupted.

“You forget, M. Poirot. We cannot risk publicity. If it were to get out that these plans had been stolen, the result would be extremely unfavourable to the Party.”

“Ah, yes,” said Poirot. “One must remember La Politique. The great discretion must be observed. You send instead for me. Ah well, perhaps it is simpler.”

“You are hopeful of success, M. Poirot?” Lord Mayfield sounded a trifle incredulous.

The little man shrugged his shoulders.

“Why not? One has only to reason—to reflect.”

He paused a moment and then said:

“I would like now to speak to Mr. Carlile.”

“Certainly.” Lord Mayfield rose. “I asked him to wait up. He will be somewhere at hand.”

He went out of the room.

Poirot looked at Sir George.

“Eh bien,” he said. “What about this man on the terrace?”

“My dear M. Poirot. Don’t ask me! I didn’t see him, and I can’t describe him.”

Poirot leaned forward.

“So you have already said. But it is a little different from that is it not?”

“What d’you mean?” asked Sir George abruptly.

“How shall I say it? Your disbelief, it is more profound.”

Sir George started to speak, then stopped.

“But yes,” said Poirot encouragingly. “Tell me. You are both at the end of the terrace. Lord Mayfield sees a shadow slip from the window and across the grass. Why do you not see that shadow?”

Carrington stared at him.

“You’ve hit it, M. Poirot. I’ve been worrying about that ever since. You see, I’d swear that no one did leave this window. I thought Mayfield had imagined it—branch of a tree waving—something of that kind. And then when we came in here and found there had been a robbery, it seemed as though Mayfield must have been right and I’d been wrong. And yet—”

Poirot smiled.

“And yet you still in your heart of hearts believe in the evidence (the negative evidence) of your own eyes?”

“You’re right, M. Poirot, I do.”

Poirot gave a sudden smile.

“How wise you are.”

Sir George said sharply: