‘Mes enfants,’ he said. ‘We must not disperse the strength. We must approach this matter with method and order in our thoughts. We must look within and not without for the truth. We must say to ourselves—each one of us—what do I know about the murderer? And so we must build up a composite picture of the man we are going to seek.’

‘We know nothing about him,’ sighed Thora Grey helplessly.

‘No, no, mademoiselle. That is not true. Each one of us knows something about him—if we only knew what it is we know. I am convinced that the knowledge is there if we could only get at it.’

Clarke shook his head.

‘We don’t know anything—whether he’s old or young, fair or dark! None of us has ever seen him or spoken to him! We’ve gone over everything we all know again and again.’

‘Not everything! For instance, Miss Grey here told us that she did not see or speak to any stranger on the day that Sir Carmichael Clarke was murdered.’

Thora Grey nodded.

‘That’s quite right.’

‘Is it? Lady Clarke told us, mademoiselle, that from her window she saw you standing on the front doorstep talking to a man.’

‘She saw me talking to a strange man?’ The girl seemed genuinely astonished. Surely that pure, limpid look could not be anything but genuine.

She shook her head.

‘Lady Clarke must have made a mistake. I never—Oh!’

The exclamation came suddenly—jerked out of her. A crimson wave flooded her cheeks.

‘I remember now! How stupid! I’d forgotten all about it. But it wasn’t important. Just one of those men who come round selling stockings—you know, ex-army people. They’re very persistent. I had to get rid of him. I was just crossing the hall when he came to the door. He spoke to me instead of ringing but he was quite a harmless sort of person. I suppose that’s why I forgot about him.’

Poirot was swaying to and fro, his hands clasped to his head. He was muttering to himself with such vehemence that nobody else said anything, but stared at him instead.

‘Stockings,’ he was murmuring. ‘Stockings…stockings…stockings…c¸a vient…stockings…stockings…it is the motif—yes…three months ago…and the other day…and now. Bon Dieu, I have it!’

He sat upright and fixed me with an imperious eye.

‘You remember, Hastings? Andover. The shop. We go upstairs. The bedroom. On a chair. A pair of new silk stockings. And now I know what it was that roused my attention two days ago. It was you, mademoiselle—’ He turned on Megan. ‘You spoke of your mother who wept because she had bought your sister some new stockings on the very day of the murder…’

He looked round on us all.

‘You see? It is the same motif three times repeated. That cannot be coincidence. When mademoiselle spoke I had the feeling that what she said linked up with something. I know now with what. The words spoken by Mrs Ascher’s next-door neighbour, Mrs Fowler. About people who were always trying to sell you things—and she mentioned stockings. Tell me, mademoiselle, it is true, is it not, that your mother bought those stockings, not at a shop, but from someone who came to the door?’