Summary. It was Dr Quill, and not for the fortune, nor to silence a woman who saw too much of other people's business, but to silence what she had proven of his own. Here is how I came to it.

The Reckoning. The room told the truth before anyone in it did. No struggle, no cloth torn, no chair overset — a killing done in a single motion by a hand that did not mean to be there at all. The lancet was the tell. It came from Dr Quill's own case, left open in the hall alcove, and the hall stood empty from half past ten till Mrs Gow's return a little after eleven. Whoever went to Nan passed through that hall, and whoever passed through it could have taken up the blade without forethought. A poisoner plans his instrument. This was a man who found one to hand.

The others fall away for the plainest of reasons — they were watched, every one of them, through the window in which Nan died. Lady Lavinia sat the cards from ten o'clock, partnered hand after hand, the Colonel and two guests ready to swear she never rose. Effie was the party's own centre, opening her presents before the whole room from ten until past eleven, never a minute unaccompanied. The Colonel himself presided over the lot of it in continuous company. None of the three had an unwatched quarter-hour in which to walk to the morning-room and back. Dr Quill alone stepped out "for air" after the cake and could be placed by no one until it was done.

The surface reading was tidy and wrong twice over. The household wanted to call it the inheritance — Nan the poor relation, co-heir in all but name, removed on the very night the money was in the air. Or they wanted to call it the medium's business, Nan silenced for knowing where the trysts were kept. Both readings mistake the kind of man Quill is. He is a frightened man, not a covetous one; what he does, he does out of panic, never out of design. And the one person in that house he had never had cause to fear was Nan — they had been easy together since childhood, unguarded as with no one else. Which is exactly why she was the one who could destroy him. The slip in her account-book, the fresh figures still wet with ink, the folded note naming a sum, a date, and the word draught — she had found his error, the sleeping-draught that nearly killed Effie's friend and that he had signed off as misfortune. She meant to put it to him plainly, as an old friend does. He took it, in his fright, as a woman about to end him, and the case in the hall was open, and his hand did the rest before his reason caught up with it. He set down her glass untouched, left the room as he'd found it but for her, and went back in to the cake and the presents as though nothing in the world had changed.

I confess I nearly overlooked the note's ink — still tacky when Mrs Gow found her, which fixes the writing and the dying within the same quarter-hour, and puts the last hand near her as his alone. It is a small thing. It closes the file all the same.

This makes the third this twelvemonth, by my count, out of that house and this one, and I do not expect it to be the last. Dr Tobias Quill is charged with the wilful killing of Nan Pringle. The rest — what she was to him, what he could not bear her knowing — is his own account to settle, and none of the court's business but the fact of it.

When Quill made his visit, the household stood still,
And Nan sat composing—what fortune, what will—
He entered the morning-room, pale with dismay,
The lancet lay open, so near and so grey,
We're taught by this tragedy (though we forget):
That knowledge, once grasped, is the deepest of debt.