t was beginning to get dark as Sarah walked towards Princes Street, the failing light hastening her stride. She had spent longer than she intended at the Infirmary. It was her policy never to be out alone after nightfall, and though she would likely be home before the lamplighter appeared on Albany Street, she did not want Archie to worry.

Fear was prominent in her mind, though. She was wary of what might lurk in each close and alley that she passed, imagined horrors that grew all the more frightening as the sun set. She thought of Nora’s fear too, that look of terror revisited as she recounted her story.

Mhairi had attributed it all to delirium, possibly an effect of the medicine Nora had been given. However, delirium by its very nature was transient, and the phantoms it summoned were as quickly dismissed. Sarah knew from her own experience that though demons could sometimes assail her in the dead of night, the fear she felt was dispelled by morning, its source instantly forgotten. The traumatic nature of the event Nora had described was evident in her retelling of it. That she still felt such fear all this time later suggested that it was born of something tangible.

But equally, Sarah could understand Mhairi’s scorn. She knew that the night nurses could be a law unto themselves. Raven had said as much on numerous occasions – poorly paid drudges who helped themselves to the laudanum on occasion and rested their weary bones in empty beds. But what would make anyone get in beside one of the patients? Thinking about some of the poor souls she had encountered in Dr Simpson’s waiting room, Sarah couldn’t imagine ever being tempted to do such a thing.

With frustration she realised that, despite this new information, she was still stuck with Raven’s seemingly intractable questions. The symptoms Nora had experienced did not match the ones described in any of Raven’s cases, but that was a mere detail in comparison with the equally obstinate issue of motive. Was Mary Dempster deliberately killing people? And if so, why?

As she approached the High Street, she saw that her route across to the North Bridge was obstructed by an overturned dray. The wagon had shed its load of beer barrels and was attracting a sizeable crowd. As she skirted round the back of the Tron Kirk, she sifted once again through the reasons people usually had for killing one another – greed, lust, envy, anger, vengeance, power – but none of them seemed to apply. Unless they applied in some obscure way that she could not see. How did killing her patients, and latterly those who employed her, profit or satisfy Mary Dempster?

This is not like some scheming wife poisoning her rich husband so that she might live off the inheritance, Raven had said.