Summary. It was Captain Wat Frew, the man who brought her opium ashore, and he killed her because she had turned the tables on him: he had been raising her price week upon week, and she had begun demanding it back and threatening to name him to the Colonel, which would have ended his trade in Cairnholm for good. Here is how I came to it.

The Reckoning. The wound is plain enough — one thrust, from behind, in a passage gone dark by design, the lamp cold at the tap. Whoever did it knew the house and had reason to want no face seen and no word exchanged. The blade came off the Colonel's own rack, one of six, from a drawer any guest might open; it tells us nothing of who took it, only that whoever did meant to leave no weapon of his own behind. The surface of the thing, and the household read it so, was money — Effie the heir-presumptive, her ledger of the Colonel's ailments in every drawer, and so either Nan the rival claimant, or the Colonel affronted at being counted for dead, or Quill silencing a patient who knew his trade too well. None of it holds. The Colonel stood at his own table before eleven men the whole of the window, and his will had not moved in four years nor was there talk of moving it. Nan was in the still-room with Mrs Gow the whole hour, counting silver under her eye. Quill never left his chair, is vouched by his neighbour, and his dealing with Effie was an open prescription, paid and written at the chemist's, unrenewed a fortnight and explaining nothing of the blue paper twist found empty by her hand. That twist is the whole of the matter. It is not a chemist's article. Turn to the memorandum page pushed into the looking-glass frame — a column of dates, eleven weeks, a price climbing from six shillings to two pounds ten, and beneath it her own hand: tell the Colonel where it comes from and it costs me nothing. That is not a debtor's ledger. That is a customer's record of being squeezed, and a threat to the one man in this parish who lands goods no customs officer has seen.

Lavinia, Fifi, Zane, Cousin R, Xerxes, Aziz — each was seen by another through the whole of the window, and seen by someone not obliged to lie for them. Wat was not. He left the table at 9:35 on the excuse of a horse and did not go near the stable — Mrs Gow, at her window the whole evening, saw no one cross the yard, and a squall had wetted it through since nine, yet his boots came back dry at 10:10. He had reason enough: three weeks since, in the harbour lane, Effie told him plainly that a price doubled could be halved again, and that she was not without friends who wrote letters — meaning the Colonel, who would break with any man found bringing worse than brandy into his house. When she went to the passage that night on what she called business, it was him she meant to meet, to have the last word on her price. He gave her the knife instead, and took the memorandum-page from her hand after — though he did not carry it out with him. Hearing Zane cross the hall, he folded it small and pushed it into the frame of the looking-glass, and went back to his dinner with dry boots and an empty excuse. I had nearly set aside the blue twist as nothing more than a dead woman's habit; it is in fact the whole of the case, being the one item at the scene that is neither a keepsake nor a coincidence, but a bill unpaid. Captain Wat Frew will answer for the killing of Effie Strange, the third this twelvemonth, by my count, and the file may close.

Young Wat took up the blade with fingers cold and lean,
Poor Effie in the passage met what lay unseen,
The lamp was turned to darkness where the shadows fell,
A memorandum crumpled that would never tell,
We ought to guard our neighbours from despair and need,
But hunger writes a logic that we heed indeed.