‘We now come to murder No. 3—the important—in fact, the real murder from your point of view.

‘And here a full meed of praise is due to Hastings, who made a simple and obvious remark to which no attention was paid.

‘He suggested that the third letter went astray intentionally!

‘And he was right!…

‘In that one simple fact lies the answer to the question that has puzzled me so all along. Why were the letters addressed in the first place to Hercule Poirot, a private detective, and not to the police?

‘Erroneously I imagined some personal reason.

‘Not at all! The letters were sent to me because the essence of your plan was that one of them should be wrongly addressed and go astray—but you cannot arrange for a letter addressed to the Criminal Investigation Department of Scotland Yard to go astray! It is necessary to have a private address. You chose me as a fairly well-known person, and a person who was sure to take the letters to the police—and also, in your rather insular mind, you enjoyed scoring off a foreigner.

‘You addressed your envelope very cleverly—Whitehaven—Whitehorse—quite a natural slip. Only Hastings was sufficiently perspicacious to disregard subtleties and go straight for the obvious!

‘Of course the letter was meant to go astray! The police were to be set on the trail only when the murder was safely over. Your brother’s nightly walk provided you with the opportunity. And so successfully had the A B C terror taken hold on the public mind that the possibility of your guilt never occurred to anyone.

‘After the death of your brother, of course, your object was accomplished. You had no wish to commit any more murders. On the other hand, if the murders stopped without reason, a suspicion of the truth might come to someone.

‘Your stalking horse, Mr Cust, had so successfully lived up to his role of the invisible—because insignificant—man, that so far no one had noticed that the same person had been seen in the vicinity of the three murders! To your annoyance, even his visit to Combeside had not been mentioned. The matter had passed completely out of Miss Grey’s head.

‘Always daring, you decided that one more murder must take place but this time the trail must be well blazed.

‘You selected Doncaster for the scene of operations.

‘Your plan was very simple. You yourself would be on the scene in the nature of things. Mr Cust would be ordered to Doncaster by his firm. Your plan was to follow him round and trust to opportunity. Everything fell out well. Mr Cust went to a cinema. That was simplicity itself. You sat a few seats away from him. When he got up to go, you did the same. You pretended to stumble, leaned over and stabbed a dozing man in the row in front, slid the A B C on to his knees and managed to collide heavily with Mr Cust in the darkened doorway, wiping the knife on his sleeve and slipping it into his pocket.