‘But not for me! I know nothing—nothing at all! Neither the why nor the wherefore.

‘And there is one small vexing fact. The man Cust has an alibi for the night of the Bexhill crime.’

‘That’s been worrying me all along,’ said Franklin Clarke.

‘Yes. It worried me. For the alibi, it has the air of being genuine. But it cannot be genuine unless—and now we come to two very interesting speculations.

‘Supposing, my friends, that while Cust committed three of the crimes—the A, C, and D crimes—he did not commit the B crime.’

‘M. Poirot. It isn’t—’

Poirot silenced Megan Barnard with a look.

‘Be quiet, mademoiselle. I am for the truth, I am! I have done with lies. Supposing, I say, that A B C did not commit the second crime. It took place, remember, in the early hours of the 25th—the day he had arrived for the crime. Supposing someone had forestalled him? What in those circumstances would he do? Commit a second murder, or lie low and accept the first as a kind of macabre present?’

‘M. Poirot!’ said Megan. ‘That’s a fantastic thought! All the crimes must have been committed by the same person!’

He took no notice of her and went steadily on:

‘Such a hypothesis had the merit of explaining one fact—the discrepancy between the personality of Alexander Bonaparte Cust (who could never have made the click with any girl)and the personality of Betty Barnard’s murderer. And it has been known, before now, that would-be murderers have taken advantage of the crimes committed by other people. Not all the crimes of Jack the Ripper were committed by Jack the Ripper, for instance. So far, so good.

‘But then I came up against a definite difficulty.

‘Up to the time of the Barnard murder, no facts about the A B C murders had been made public. The Andover murder had created little interest. The incident of the open railway guide had not even been mentioned in the press. It therefore followed that whoever killed Betty Barnard must have had access to facts known only to certain persons—myself, the police, and certain relations and neighbours of Mrs Ascher.

‘That line of research seemed to lead me up against a blank wall.’

The faces that looked at him were blank too. Blank and puzzled.

Donald Fraser said thoughtfully:

‘The police, after all, are human beings. And they’re good-looking men—’

He stopped, looking at Poirot inquiringly.

Poirot shook his head gently.

‘No—it is simpler than that. I told you that there was a second speculation.

‘Supposing that Cust was not responsible for the killing of Betty Barnard? Supposing that someone else killed her. Could that someone else have been responsible for the other murders too?’

‘But that doesn’t make sense!’ cried Clarke.

‘Doesn’t it? I did then what I ought to have done at first. I examined the letters I had received from a totally different point of view. I had felt from the beginning that there was something wrong with them—just as a picture expert knows a picture is wrong…