As he did not answer, she repeated urgently:
“Will you guarantee that there will be no publicity?”
He answered then—very gravely:
“Yes, madame, I will guarantee that.”
“Then everything can be arranged.”
She passed abruptly from the room. A moment later Poirot heard the car drive away.
He crossed the hall and went along the passage to the study. Lord Mayfield was there. He looked up as Poirot entered.
“Well?” he said.
Poirot spread out his hands.
“The case is ended, Lord Mayfield.”
“What?”
Poirot repeated word for word the scene between himself and Lady Julia.
Lord Mayfield looked at him with a stupefied expression.
“But what does it mean? I don’t understand.”
“It is very clear, is it not? Lady Julia knows who stole the plans.”
“You don’t mean she took them herself?”
“Certainly not. Lady Julia may be a gambler. She is not a thief. But if she offers to return the plans, it means that they were taken by her husband or her son. Now Sir George Carrington was out on the terrace with you. That leaves us the son. I think I can reconstruct the happenings of last night fairly accurately. Lady Julia went to her son’s room last night and found it empty. She came downstairs to look for him, but did not find him. This morning she hears of the theft, and she also hears that her son declares that he went straight to his room and never left it. That, she knows, is not true. And she knows something else about her son. She knows that he is weak, that he is desperately hard up for money. She has observed his infatuation for Mrs. Vanderlyn. The whole thing is clear to her. Mrs. Vanderlyn has persuaded Reggie to steal the plans. But she determines to play her part also. She will tackle Reggie, get hold of the papers and return them.”
“But the whole thing is quite impossible,” cried Lord Mayfield.
“Yes, it is impossible, but Lady Julia does not know that. She does not know what I, Hercule Poirot, know, that young Reggie Carrington was not stealing papers last night, but instead was philandering with Mrs. Vanderlyn’s French maid.”
“The whole thing is a mare’s nest!”
“Exactly.”
“And the case is not ended at all!”
“Yes, it is ended. I, Hercule Poirot, know the truth. You do not believe me? You did not believe me yesterday when I said I knew where the plans were. But I did know. They were very close at hand.”
“Where?”
“They were in your pocket, my lord.”
There was a pause, then Lord Mayfield said:
“Do you really know what you are saying, M. Poirot?”
“Yes, I know. I know that I am speaking to a very clever man. From the first it worried me that you, who were admittedly shortsighted, should be so positive about the figure you had seen leaving the window. You wanted that solution—the convenient solution—to be accepted. Why? Later, one by one, I eliminated everyone else. Mrs. Vanderlyn was upstairs, Sir George was with you on the terrace, Reggie Carrington was with the French girl on the stairs, Mrs. Macatta was blamelessly in her bedroom. (It is next to the housekeeper’s room, and Mrs. Macatta snores!) Lady Julia clearly believed her son guilty. So there remained only two possibilities. Either Carlile did not put the papers on the desk but into his own pocket (and that is not reasonable, because, as you pointed out, he could have taken a tracing of them), or else—or else the plans were there when you walked over to the desk, and the only place they could have gone was into your pocket. In that case everything was clear. Your insistence on the figure you had seen, your insistence on Carlile’s innocence, your disinclination to have me summoned.
“One thing did puzzle me—the motive. You were, I was convinced, an honest man, a man of integrity. That showed in your anxiety that no innocent person should be suspected. It was also obvious that the theft of the plans might easily affect your career unfavourably. Why, then, this wholly unreasonable theft? And at last the answer came to me. The crisis in your career, some years ago, the assurances given to the world by the Prime Minister that you had had no negotiations with the power in question. Suppose that that was not strictly true, that there remained some record—a letter, perhaps—showing that in actual fact you had done what you had publicly denied. Such a denial was necessary in the interests of public policy. But it is doubtful if the man in the street would see it that way. It might mean that at the moment when supreme power might be given into your hands, some stupid echo from the past would undo everything.
“I suspect that that letter has been preserved in the hands of a certain government, that that government offered to trade with you—the letter in exchange for the plans of the new bomber. Some men would have refused. You—did not! You agreed. Mrs. Vanderlyn was the agent in the matter. She came here by arrangement to make the exchange. You gave yourself away when you admitted that you had formed no definite stratagem for entrapping her. That admission made your reason for inviting her here incredibly weak.