“Whilst you were standing at the foot of the stairs here, could anyone have entered the study by the door you had left open?”

Carlile shook his head.

“Not without passing me. The study door is at the end of the passage, as you see.”

Poirot nodded thoughtfully. Mr. Carlile went on in his careful, precise voice.

“I may say that I am very thankful that Lord Mayfield actually saw the thief leaving the window. Otherwise I myself should be in a very unpleasant position.”

“Nonsense, my dear Carlile,” broke in Lord Mayfield impatiently. “No suspicion could possibly attach to you.”

“It is very kind of you to say so, Lord Mayfield, but facts are facts, and I can quite see that it looks badly for me. In any case I hope that my belongings and myself may be searched.”

“Nonsense, my dear fellow,” said Mayfield.

Poirot murmured:

“You are serious in wishing that?”

“I should infinitely prefer it.”

Poirot looked at him thoughtfully for a minute or two and murmured, “I see.”

Then he asked:

“Where is Mrs. Vanderlyn’s room situated in regard to the study?”

“It is directly over it.”

“With a window looking out over the terrace?”

“Yes.”

Again Poirot nodded. Then he said:

“Let us go to the drawing room.”

Here he wandered round the room, examined the fastenings of the windows, glanced at the scorers on the bridge table and then finally addressed Lord Mayfield.

“This affair,” he said, “is more complicated than it appears. But one thing is quite certain. The stolen plans have not left this house.”

Lord Mayfield stared at him.

“But, my dear M. Poirot, the man I saw leaving the study—”

“There was no man.”

“But I saw him—”

“With the greatest respect, Lord Mayfield, you imagined you saw him. The shadow cast by the branch of a tree deceived you. The fact that a robbery occurred naturally seemed a proof that what you had imagined was true.”

“Really, M. Poirot, the evidence of my own eyes—”

“Back my eyes against yours any day, old boy,” put in Sir George.

“You must permit me, Lord Mayfield, to be very definite on that point. No one crossed the terrace to the grass.”

Looking very pale and speaking stiffly, Mr. Carlile said:

“In that case, if M. Poirot is correct, suspicion automatically attaches itself to me. I am the only person who could possibly have committed the robbery.”

Lord Mayfield sprang up.

“Nonsense. Whatever M. Poirot thinks about it, I don’t agree with him. I am convinced of your innocence, my dear Carlile. In fact, I’m willing to guarantee it.”

Poirot murmured mildly:

“But I have not said that I suspect M. Carlile.”

Carlile answered:

“No, but you’ve made it perfectly clear that no one else had a chance to commit the robbery.”

“Du tout! Du tout!”

“But I have told you nobody passed me in the hall to get to the study door.”