Summary. It was Fifi Swain, the housemaid, and she did it to stop a letter — here is how I came to it.

The Reckoning. Nan Pringle drank a servants'-hall cup, not the drawing-room ware; that detail settled where to look before it settled who to charge. The drawing-room party had taken its own tray before the sitting began, so the cup at Nan's elbow had come from below, on the servants' errands, in the hour between half past eight and nine when the circle sat dark and unbroken. Everyone above stairs could account for that hour twice over. The Colonel and Xerxes held the head of the table, vouched each by the other's sitters; Aziz stood with the carriage in the front hall, seen by the driver and the under-footman; Lavinia's hands were held fast on both sides the whole while, as her calling requires; Quill sat with the decanter, a footman his witness; Cousin R made his slow round of the gallery under two servants' eyes; Wat waited on the boot-boy in the gun-room. None of these could have carried a tray up the back stair without being missed, and none had cause to. That clears the circle entire, and clears Quill of the poisoner's art besides — his draughts were for his own throat that evening, not Nan's.

Below stairs the field narrows fast. Zane was sent to the cellar and the front hall on Mrs Gow's own errands, placed there by her word and the driver's; the footman's hat left on the landing was meant to point at him, or at some man of his place, but a hat left where no man stood proves only whose absence someone wished remembered. That leaves the maid. Fifi's movements in the tea-tray hour rested on no word but her own, and hers was the hand that made up the servants'-hall tray while Mrs Gow's back was to the range. She dosed the cup in that gap, carried it up the back stair herself, and set it before Nan, who had no reason to fear a housemaid bringing tea and so drank it down.

As for why: I had first to set aside what did not fit. There was no long grudge here — nothing in the house's memory suggests Fifi ever spent a thought on Nan one way or the other; the woman was beneath her notice, a poor relation who kept to her notes and her corner. Nor was this about some staff-tryst at the back lane, whatever a footman's absence and a lady's carriage might seem to promise — that is a different sort of secret, and not the one that killed. What altered the balance was Nan's own habit of watching and writing down. Her notes had found the truth of what the twins were before they came up into the Colonel's service, and her letter, dried mid-sentence on the escritoire, shows her already carrying that truth to him — "a name that is not the name they give." What that name was, and what the twins had been before they wore the one they answer to now, is their own history, and I set it down no further than that; it is enough to say the Colonel's good word had secured them a place built on an untruth, and Nan meant, in her small careful hand, to unbuild it. Fifi did not kill from hatred. She killed because the one hour the house was blind was also the one hour left to stop that letter finishing, and she took it. The hat on the landing was her only misstep — an instinct to throw the thing onto a man, thrown down in haste and left to be read the wrong way round.

The charge is laid against Fifi Swain, for the wilful poisoning of Nan Pringle. The file may close.

The parlor maid named Fifi sealed her fate,
When Nan sat down to seal and to await,
A tray came up the backstairs dark and still,
One cup among them harbored bitter will,
The moral stands (though nobody shall heed):
That loose lips grow from meddlesome seed.