Colonel Carbury said: "D'you mean to tell me that one of those Bedouin fellows of mine murdered an old lady by sticking her with a hypodermic? Fantastic!"

"Wait, Colonel Carbury; I have not yet finished. Let us agree that the Arab might have come from Dr. Gerard's tent and not Ginevra Boynton's. What is the next thing? Both ladies agree that they could not see his face clearly enough to identify him and that they did not hear what was said. That is understandable. The distance between the marquee and the ledge is about two hundred yards. Lady Westholme gave a clear description of the man otherwise, describing in detail his ragged breeches and the untidiness with which his puttees were rolled."

Poirot leaned forward. "And that, my friends, was very odd indeed! Because, if she could not see his face or hear what was said, she could not possibly have noticed the state of his breeches and puttees! Not at two hundred yards!"

"It was an error, that, you see! It suggested a curious idea to me. Why insist so on the ragged breeches and untidy puttees. Could it be because the breeches were not torn and the puttees were non-existent? Lady Westholme and Miss Pierce both saw the man-but from where they were sitting they could not see each other. That is shown by the fact that Lady Westholme came to see if Miss Pierce was awake and found her sitting in the entrance of her tent."

"Good Lord," said Colonel Carbury, suddenly sitting up very straight. "Are you suggesting-"

"I am suggesting that having ascertained just what Miss Pierce (the only witness likely to be awake) was doing, Lady Westholme returned to her tent, put on her riding breeches, boots and khaki-colored coat, made herself an Arab headdress with her checked duster and a skein of knitting wool and that, thus attired, she went boldly up to Dr. Gerard's tent, looked in his medicine chest, selected a suitable drug, took the hypodermic, filled it and went boldly up to her victim."

"Mrs. Boynton may have been dozing. Lady Westholme was quick. She caught her by the wrist and injected the stuff. Mrs. Boynton half cried out-tried to rise-then sank back. The 'Arab' hurried away with every evidence of being ashamed and abashed. Mrs. Boynton shook her stick, tried to rise, then fell back into her chair."

"Five minutes later Lady Westholme rejoins Miss Pierce and comments on the scene she has just witnessed, impressing her own version of it on the other. Then they go for a walk, pausing below the ledge where Lady Westholme shouts up to the old lady. She receives no answer for Mrs. Boynton is dead but she remarks to Miss Pierce: 'Very rude just to snort at us like that!' Miss Pierce accepts the suggestion. She has often heard Mrs. Boynton receive a remark with a snort-she will swear quite sincerely if necessary that she actually heard it. Lady Westholme has sat on committees often enough with women of Miss Pieree's type to know exactly how her own eminence and masterful personality can influence them. The only point where her plan went astray was the replacing of the syringe. Dr. Gerard returning so soon upset her scheme. She hoped he might not have noticed its absence, or might think he had overlooked it, and she put it back during the night."

He stopped.

Sarah said: "But why? Why should Lady Westholme want to kill old Mrs. Boynton?"

"Did you not tell me that Lady Westholme had been quite near you in Jerusalem when you spoke to Mrs. Boynton? It was to Lady Westholme that Mrs. Boynton's words were addressed. 'I've never forgotten anything, not an action, not a name, not a face.' Put that with the fact that Mrs. Boynton had been a wardress in a prison and you can get a very shrewd idea of the truth. Lord Westholme met his wife on a voyage back from America. Lady Westholme, before her marriage, had been a criminal and had served a prison sentence."

"You see the terrible dilemma she was in? Her career, her ambitions, her social position-all at stake! What the crime was for which she served a sentence in prison we do not yet know (though we soon shall) but it must have been one that would effectually blast her political career if it was made public. And remember this, Mrs. Boynton was not an ordinary blackmailer. She did not want money. She wanted the pleasure of torturing her victim for a while and then she would have enjoyed revealing the truth in the most spectacular fashion! No; while Mrs. Boynton lived Lady Westholme was not safe. She obeyed Mrs. Boynton's instructions to meet her at Petra (I thought it strange all along that a woman with such a sense of her own importance as Lady Westholme should have preferred to travel as a mere tourist), but in her own mind she was doubtless revolving ways and means of murder. She saw her chance and carried it out boldly. She only made two slips. One was to say a little too much-the description of the torn breeches-which first drew my attention to her, and the other was when she mistook Dr. Gerard's tent and looked first into the one where Ginevra was lying half asleep. Hence the girl's story-half make-believe, half true-of a Sheikh in disguise. She put it the wrong way around, obeying her instinct to distort the truth by making it more dramatic, but the indication was quite significant enough for me."

He paused. "But we shall soon know. I obtained Lady Westholme's fingerprints today without her being aware of the fact. If these are sent to the prison where Mrs. Boynton was once a wardress, we shall soon know the truth when they are compared with the files."

He stopped. In the momentary stillness a sharp sound was heard.

"What's that?" asked Dr. Gerard.

"Sounded like a shot to me," said Colonel Carbury, rising to his feet quickly. "In the next room. Who's got that room, by the way?"

Poirot murmured: "I have a little idea-it is the room of Lady Westholme…"