hat she had only been with her mistress for two months。 Certainly it did not seem as if she could have had anything to do with the crime since she had been left behind in Paris and Mrs Kettering had been seen alive by several people afterwards； but …〃
Poirot leant forward。 He raised an emphatic forefinger and wagged it with intense emphasis at Van Aldin。
〃But I am a good detective。 I suspect。 There is nobody and nothing that I do not suspect。 I believe nothing that I am told。 I say to myself： how do we know that Ada Mason was left behind in Paris？ And at first the anspletely satisfactory。 There was the evidence of your secretary； Major Knighton； a plete outsider whose testimony might be supposed to be entirely impartial； and there was the dead woman's own words to the conductor on the train。 But I put the latter point aside for the moment； because a very curious idea … an idea perhaps fantastic and impossible … was growing up in my mind。 If by any outside chance it happened to be true； that particular piece of testimony was worthless。
〃I concentrated on the chief stumbling…block to my theory。 Major Knighton's statement that he saw Ada Mason at the Ritz after the Blue Train had left Paris。 That seemed conclusive enough； but yet； on examining the facts carefully； I noted two things。 First； that by a curious coincidence he； too； had been exactly two months in your service。 Secondly； his initial letter was the same … 'K。' Supposing … just supposing … that it was his cigarette case which had been found in the carriage。 Then； if Ada Mason and he were working together； and she recognized it when we showed it to her； would she not act precisely as she had done？ At first； taken aback； she quickly evolved a plausible theory that would agree with Mr Kettering's guilt。 Bien entendu； that was not the original idea。 The te de la Roche was to be the scapegoat； though Ada Mason would not make her recognition of him too certain； in case he should be able to prove an alibi。 Now； if you will cast your mind back to that time； you will remember a significant thing that happened。 I suggested to Ada Mason that the man she had seen was not the te de la Roche； but Derek Kettering。 She seemed uncertain at the time； but after I had got back to my hotel you rang me up and told me that she had e to you and said that； on thinking it over； she was now quite convinced that the man in question was Mr Kettering。 I had been expecting something of the kind。 There could be but one explanation of this sudden certainty on her part。 After my leaving your hotel； she had had time to consult with somebody； and had received instructions which she acted upon。 Who had given her these instructions？ Major Knighton。 And there was another very small point； which might mean nothing or might mean a great deal。 In casual conversation Knighton had talked of a jewel robbery in Yorkshire in a house where he was staying。 Perhaps a mere coincidence … perhaps another small link in the chain。〃
〃But there is one thing I do not understand； Monsieur Poirot。 I guess I must be dense or I would have seen it before now。 Who was the man in the train at Paris？ Derek Kettering or the te de la Roche？〃
〃That is the simplicity of the whole thing。 There was no man。 Ah … mille tonnerres！ … do you not see the cleverness of it all？ Whose word have we for it that there ever was a man there？ Only Ada Mason's。 And we believe in Ada Mason because of Knighton's evidence that she was left behind in Paris。〃
〃But Ruth herself told the conductor that she had left her maid behind there；〃 demurred Van Aldin。
〃Ah！ I am ing to that。 We have Mrs Kettering's own evidence there； but； on the other hand； we have not really got her evidence； because； Monsieur Van Aldin； a dead woman cannot give evidence。 It is not her evidence； but the evidence of the conductor of the train … a very different affair altogether。〃
〃So you think the man was lying？〃
〃No； no； not at all。 He spoke what he thought to be the truth。 But the woman who told him that she had left her maid in Paris was not Mrs Kettering。〃
Van Aldin stared at him。
〃Monsieur Van Aldin； Ruth Kettering was dead before the train arrived at the Gare de Lyon。 It was Ada Mason； dressed in her mistress's very distinctive clothing； who purchased a dinner basket and who made that very necessary statement to the conductor。〃
〃Impossible！〃
〃No； no； Monsieur Van Aldin； not impossible。 Les femmes； they look so much alike nowadays that one identifies them more by their clothing than by their faces。 Ada Mason was the same height as your daughter。 Dressed in that very sumptuous fur coat and the little red lacquer hat jammed down over her eyes； with just a bunch of auburn curls showing over each ear； it was no wonder that the conductor was deceived。 He had not previously spoken to Mrs Kettering； you remember。 True； he had seen the maid just for a moment when she handed him the tickets； but his impression had been merely that of a gaunt； black…clad female。 If he had been an unusually intelligent man； he might have gone so far as to say that mistress and maid were not unlike； but it is extremely unlikely that he would even think that。 And remember； Ada Mason； or Kitty Kidd； was an actress； able to change her appearance and tone of voice at a moment's notice。 No； no； there was no danger of his recognizing the maid in the mistress's clothing； but there was the danger that when he came to discover the body he might realize it was not the woman he had talked to the night before。 And now we see the reason for the disfigured face。 The chief danger that Ada Mason ran was that Katherine Grey might visit her partment after the train left Paris； and she provided against that difficulty by ordering a dinner basket and by locking herself in her partment。〃
〃But who killed Ruth … and when？〃
〃First； bear it in mind that the crime was planned and undertaken by the two of them … Knighton and Ada Mason； working together。 Knighton was in Paris that day on your business。 He boarded the train somewhere on its way round the ceinture。 Mrs Kettering would be surprised； but she would be quite unsuspicious。 Perhaps he draws her attention to something out the window； and as she turns to look he slips the cord round her neck … and the whole thing is over in a second or two。 The door of the partment is locked； and he and Ada Mason set to work。 They strip off the dead woman's outer clothes。 Mason and Knighton roll the body up in a rug and put it on the seat in the adjoining partment amongst the bags and suitcases。 Knighton drops off the train； taking the jewel…case containing the rubies with him。 Since the crime is not supposed to have been mitted until nearly twelve hours later he is perfectly safe； and his evidence and the supposed Mrs Kettering's words to the conductor will provide a perfect alibi for his acplice。
〃At the Gare de Lyon Ada Mason gets a dinner basket； and shutting herself into the toilet partment she quickly changes into her mistress's clothes； adjusts two false bunches of auburn curls； and generally makes up to resemble her as closely as possible。 When the conductor es to make up the bed； she tells him the prepared story about having left her maid behind in Paris； and whilst he is making up the berth； she stands looking out of the window； so that her back is towards the corridor and people passing along there。 That was a wise precaution； because； as we know； Miss Grey was one of those passing； and she among others； was willing to swear that Mrs Kettering was still alive at that hour。〃
〃Go on；〃 said Van Aldin。
〃Before getting to Lyons； Ada Mason arranged her mistress's body in the bunk； folded up the dead woman's clothes neatly on the end of it； and herself changed into a man's clothes and prepared to leave the train。 When Derek Kettering entered his wife's partment； and； as he thought； saw her asleep in her berth； the scene had been set； and Ada Mason was hidden in the next partment waiting for the moment to leave the train unobserved。 As soon as the conductor had swung himself down on to the platform at Lyons； she follows； slouching along as though just taking a breath of air。 At a moment when she is unobserved； she hurriedly crosses to the other platform； and takes the first train back to Paris and the Ritz Hotel。 Her name has been registered there as taking a room the night before by one of Knighton's female acplices。 She has nothing to do but wait there placidly for your arrival。 The jewels are not； and never have been； in her possession。 Kettering had them。 No suspicion attaches to him； and； as your secretary； he brings them to Nice without the least fear of discovery。 Their delivery there to Monsieur Papopolous is already arranged for and they are entrusted to Mason at the last moment to hand over to the Greek。 Altogether a very neatly planned coup； as one would expect from a master of the game such as the Marquis。〃
〃And you honestly mean that Richard Knighton is a well…known criminal； who has been at this business for years？〃
Poirot nodded。
〃One of the chief assets of the gentleman called the Marquis was his plausible； ingratiating manner。 You fell a victim to his charm； Monsieur Van Aldin； when you engaged him as a secretary on such a slight acquaintanceship？〃
〃I could have sworn that he never angled for the post；〃 cried the millionaire。
〃It was very astutely done … so astutely done that it deceived a man whose knowledge of other men is as great as yours is。〃
〃I looked up his antecedents too。 The fellow's record was excellent。〃
〃Yes； yes； that was part of the game。 As Richard Knighton his life reproach。 He was well…born； well…connected； did honourable service in the war； and seemed altogether above suspicion； but when I came to glean information about the mysterious Marquis； I found many points of similarity。 Knighton spoke French like a Frenchman； he had been in America； France； and England at much the same time as the Marquis was operating。 The Marquis was last heard of as engineering various jewel robberies in Switzerland； and it was in Switzerland that you had e across Major Knighton； and it was at precisely that time that the first rumours were going round of your being in treaty for the famous rubies。〃
〃But why murder？〃 murmured Van Aldin brokenly。 〃Surely a clever thief could have stolen the jewels without running his head into a noose。〃
Poirot shook his head。 〃This is not the first murder that lies to the Marquis's charge。 He is a killer by instinct； he believes； too； in leaving no evidence behind him。 Dead men and women tell no tales。
〃The Marquis had an intense passion for famous and historical jewels。 He laid his plans far beforehand by installing himself as your secretary and getting his acplice to obtain the situation of maid with your daughter； for whom he guessed the jewels were destined。 And； though this was his matured and carefully thought…out plan； he did not scruple to attempt a short…cut by hiring a couple of Apaches to waylay you in Paris on the night you bought the jewels。 That plan failed； which hardly surprised him； I think。 This plan was； so he thought； pletely safe。 No possible suspicion could attach to Richard Knighton。 But like all great men … and the Marquis was a great