“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?”