Curtain: Poirot's Last Case: A Hercule Poirot Mystery: The Official Authorized Edition

by Agatha Christie

Apart from a person connected with the police force or with, say, a firm of criminal lawyers, it is not reasonable for any man or woman to be involved in five murder cases. It does not, you comprehend, happen! Never, never does it occur that someone says confidentially: ‘Well, as a matter of fact, I’ve actually known five murderers!’ No, no, mon ami, it is not possible, that. So we get the curious result that we have here a case of catalysis—a reaction between two substances that takes place only in the presence of a third substance, that third substance apparently taking no part in the reaction and remaining unchanged. That is the position. It means that where X was present, crimes took place—but X did not actively take part in these crimes. “An extraordinary, an abnormal situation! And I saw that I had come across at last, at the end of my career, the perfect criminal, the criminal who had invented such a technique that he could never be convicted of crime.