Summary. It was Nan, the Colonel's niece, and she did it to keep herself unmarried; here is how I came to it.

The Reckoning. The cause was poison, and the poison went into one glass only — the Colonel's own. Dr Quill's testing cleared the bottle and every other vessel on the board; whatever killed Cecil Cruikshank was placed by a hand that knew which glass was his and had leave to touch it without remark. That narrows the field a good deal before we so much as reach for motive. The commotion itself, I should say, was no accident of nature. Catnip does not find its way fresh-cut into a salad by chance, and the bed it came from grows in the Manor's own garden — tended, watered, and cut by the same person who keeps that household's accounts and its kitchen both. The cheetah's turn upon the table was stagecraft, got up to draw every eye and every hand from the head of the table for the one half-minute required.

The others fall away easily enough, and I will not linger over them. Xerxes sat as co-host in plain view, and had, besides, no reason to hasten a death that only brought his own marriage nearer, not further off — the venture's founding was the very seal on the match the Colonel was making for him. Aziz was seen the whole time chasing his mistress's cat among the candlesticks, nowhere near the Colonel's place. Zane and Fifi were accounted for, belowstairs and at table both, by Mrs Gow, and had, in any case, nothing threatened that night to drive them to it. Wat was drunk at the quay and nowhere near the table at all. Cousin R sat a guest with no reach to the glass, and had every interest in the Colonel living, being not yet done with him. Lavinia's purse depended on the Colonel's continuing patronage of her séances, so his death served her no better than it served Effie, whose expectation under an unsettled will was cut off, not advanced, by his sudden end. Quill himself called for the very testing that isolated the single poisoned glass — no poisoner asks so plainly to be found out.

That leaves the niece who sat at the Colonel's hand as she always did, who saw to his glass and his place as a matter of course, unremarked because unremarkable — until one asks why a woman with nothing to gain by the venture's failure should wish it to fail. She had everything to gain, in fact, by his death, and nothing at all by his living another week. The venture's founding was to seal, that same evening, an arranged match between herself and Xerxes Hartfield — pressed upon her by the Colonel himself, who was, being her late aunt's kin and her sole means of keeping herself, the only man whose word could make or unmake such a bond for her. Once the venture was founded the match was as good as sealed, and no door left open. His death, coming before the seal was set, closed the venture's business and opened, for her, the one door out of the marriage she could not otherwise have refused. A small thing very nearly slipped past me: the dirt on her shoe, and the letter she said she'd had to post that afternoon — offered before anyone asked. I make no more of it than this, that a woman rehearsing her excuses in advance of being questioned is a woman who has already reckoned she may need them. I have written to have her held. The venture, I understand, will not now be founded at all.

The hand that mixed the poison came from Nan,
The Colonel drank, and Nan's own plan began
To flower in the chaos and the noise
While every soul had lost their evening's joys;
We learn that nieces smile through gritted teeth,
That hunger wears the mask of mourning's wreath.