‘Go on,’ said the girl.

‘To begin with,’ said Poirot, ‘there was the smell of cigarette smoke.’

‘I think you’re exaggerating there, Poirot,’ said Japp. ‘I didn’t smell anything.’

Poirot turned on him in a flash.

‘Precisely. You did not smell any stale smoke. No more did I. And that was very, very strange—for the door and the window were both closed and on an ashtray there were the stubs of no fewer than ten cigarettes. It was odd, very odd, that the room should smell—as it did, perfectly fresh.’

‘So that’s what you were getting at!’ Japp sighed. ‘Always have to get at things in such a tortuous way.’

‘Your Sherlock Holmes did the same. He drew attention, remember, to the curious incident of the dog in the night-time—and the answer to that was there was no curious incident. The dog did nothing in the night-time. To proceed:

‘The next thing that attracted my attention was a wrist-watch worn by the dead woman.’

‘What about it?’

‘Nothing particular about it, but it was worn on the right wrist. Now in my experience it is more usual for a watch to be worn on the left wrist.’

Japp shrugged his shoulders. Before he could speak, Poirot hurried on:

‘But as you say, there is nothing very definite about that. Some people prefer to wear one on the right hand. And now I come to something really interesting—I come, my friends, to the writing-bureau.’

‘Yes, I guessed that,’ said Japp.

‘That was really very odd—very remarkable! For two reasons. The first reason was that something was missing from that writing-table.’

Jane Plenderleith spoke.

‘What was missing?’

Poirot turned to her.

‘A sheet of blotting-paper, mademoiselle. The blotting-book had on top a clean, untouched piece of blotting-paper.’

Jane shrugged her shoulders.

‘Really, M. Poirot. People do occasionally tear off a very much used sheet!’

‘Yes, but what do they do with it? Throw it into the waste-paper basket, do they not? But it was not in the waste-paper basket. I looked.’

Jane Plenderleith seemed impatient.

‘Because it had probably been already thrown away the day before. The sheet was clean because Barbara hadn’t written any letters that day.’

‘That could hardly be the case, mademoiselle. For Mrs Allen was seen going to the post-box that evening. Therefore she must have been writing letters. She could not write downstairs—there were no writing materials. She would be hardly likely to go to your room to write. So, then, what had happened to the sheet of paper on which she had blotted her letters? It is true that people sometimes throw things in the fire instead of the waste-paper basket, but there was only a gas fire in the room. And the fire downstairs had not been alight the previous day, since you told me it was all laid ready when you put a match to it.’

He paused.

‘A curious little problem. I looked everywhere, in the waste-paper baskets, in the dustbin, but I could not find a sheet of used blotting-paper—and that seemed to me very important. It looked as though someone had deliberately taken that sheet of blotting paper away. Why? Because there was writing on it that could easily have been read by holding it up to a mirror.