Poirot smiled.
"It is very confusing, this affair of the hypodermic - and yet, I think, it could be explained.
Ah, well, we have now two cases made out - the case for the innocence of Raymond Boynton - the case for the guilt of his sister Carol.
But me, I am scrupulously fair. I look always on both sides.
Let us examine what occurred if Carol Boynton was innocent."
"She returns to the camp, she goes up to her stepmother, and she finds her - shall we say - dead!
What is the first thing she will think?
She will suspect that her brother Raymond may have killed her.
She does not know what to do.
So she says nothing.
And presently, about an hour later, Raymond Boynton returns and, having presumably spoken to his mother, says nothing of anything being amiss.
Do you not think that then her suspicions would become certainties?
Perhaps she goes to his tent and finds there a hypodermic syringe.
Then, indeed she is sure!
She takes it quickly and hides it. Early in the morning she flings it as far away as she can."
"There is one more indication that Carol Boynton is innocent.
She assures me, when I question her, that she and her brother never seriously intended to carry out their plan.
I ask her to swear - and she swears immediately and with the utmost solemnity that she is not guilty of the crime! You see, that is the way she puts it. She does not swear that they are not guilty.
She swears for herself, not her brother - and thinks that I will not pay special attention to the pronoun."
"Eh bien, that is the case for the innocence of Carol Boynton.
And now let us go back a step and consider not the innocence but the possible guilt of Raymond.
Let us suppose that Carol is speaking the truth, that Mrs. Boynton was alive at five-ten.
Under what circumstances can Raymond be guilty?
We can suppose that he killed his mother at ten minutes to six when he went up to speak to her.
There were boys about the camp, true, but the light was failing.
It might have been managed but it then follows that Miss King lied.
Remember, she came back to the camp only five minutes after Raymond. From the distance she would see him go up to his mother.
Then, when later she is found dead, Miss King realizes that Raymond has killed her. To save him, she lies - knowing that Dr. Gerard is down with fever and cannot expose her lie!"
"I did not lie!" said Sarah clearly.
"There is yet another possibility.
Miss King, as I have said, reached the camp a few minutes after Raymond.
If Raymond Boynton found his mother alive, it may have been Miss King who administered the fatal injection.
She believed that Mrs. Boynton was fundamentally evil. She may have seen herself as a just executioner.
That would equally well explain her lying about the time of death."
Sarah had grown very pale. She spoke in a low steady voice:
"It is true that I spoke of the expediency of one person dying to save many.
It was the Place of Sacrifice that suggested the idea to me.
But I can swear to you that I never harmed that disgusting old woman - nor would the idea of doing so ever have entered my head!"
"And yet," said Poirot softly, "one of you two must be lying."
Raymond Boynton shifted in his chair. He cried out impetuously: "You win, M. Poirot! I'm the liar.
Mother was dead when I went up to her.
It - it quite knocked me out.
You see, I'd been going to have it out with her. To tell her that from henceforth I was a free agent. I was all set, you understand. And there she was - dead!
Her hand all cold and flabby. And I thought - just what you said. I thought maybe Carol - you see, there was the mark on her wrist - "
Poirot said quickly: "That is the one point on which I am not yet completely informed. What was the method you counted on employing?
You had a method - and it was connected with a hypodermic syringe.
That much I know. If you want me to believe you, you must tell me the rest."
Raymond said hurriedly: "It was a way I read in a book - an English detective story. You stuck an empty hypodermic syringe into someone and it did the trick.
It sounded perfectly scientific.
I - I thought we'd do it that way."