She was making her way back up from Stockbridge, where she had spoken to a Mrs Haldane, who told her how Mary Dempster had nursed her sister back to health after a bout of enteric fever. Sarah had concocted a story about trying to find the nurse to give her a gift bequeathed by a recently departed relative, in gratitude for services rendered. Mrs Haldane was effusive in her praise of Mary, hardly supporting their theory that she was in fact a ruthless killer.

Before that, however, she had spoken to a Mrs Guthrie, who was the cook in a house where Mary had also been employed. The lady of the house, Mrs Bennet, was from home and in her absence, Mrs Guthrie told Sarah how Mrs Bennet’s ‘illness’ was in fact a result of her late husband being quick with his fists. She had recovered under Mary’s care, but her husband had fallen ill and died shortly afterwards.

Mrs Guthrie’s description of his deterioration was familiar. A sudden illness to which he quickly succumbed. Delirium, coma and death in rapid succession. This brought her back to the unresolved issue of how this might have been achieved without arousing suspicion. Why kill some patients and not others, and why risk killing so many in one family, as in the Trinity case? The questions continued to mount up and they seemed to be no closer to answering any of them.

As she ascended the slope of Dublin Street, around the corner from her house, she passed a grocer’s shop which had a variety of vegetables displayed in the window. She stopped to look, giving herself a moment to catch her breath.

She thought about Mrs Lyndsay’s cooking and hoped that Archie would be tempted to eat a little of it. A forlorn hope perhaps. It was unlikely that he would have the strength to sit through meals with the rest of the family.

She thought too of Mrs Lyndsay’s resistance to change, her adherence to the established order of things. Like her recipes, she understood what worked and feared altering any of the components. She seemed to think that the whole of society worked that way too.

There was a pile of potatoes in the middle of the window display. Sarah was reminded of the garden at Lochend and the plant that she could not identify. The plant that was still flowering so late in the season. The plant that Martha had said was potatoes. She had accepted Martha’s suggestion but now she realised that Martha had been wrong. The plant was not potatoes, though it did look similar, because it was from the same family.

It was deadly nightshade. Atropa belladonna. Atropine.

She saw now why they could not identify the poison Mary was using from the symptoms being exhibited. It was because she was using more than one.