He had no other reason to be grateful for the murk, as it was a considerable impediment to his navigation, obscuring the landmarks by which he would normally get his bearings. He wasted a good ten minutes walking up the wrong road, taking a turn too soon. Then, once he had doubled back and corrected his course, he realised he had walked past the cottage he was looking for, as it was set back from the road.

His approach being obscured, he wondered whether he might benefit from catching his quarry unawares. Sarah had spoken of her suspicion that Mary had been in the house when last they visited. She thought she had seen someone at the window, and there had been a delay in Martha answering the door: long enough for Mary to instruct her sister to conceal that she was home.

The question this posed, however, was how Mary would have known to be evasive of two strangers. Then Raven remembered that she had seen him, albeit briefly, at the Porteous house. It was therefore possible that she had spied him from the window, and though she could not extrapolate from that the reason he was there, she had not evaded discovery for this long by being careless and taking chances.

The front door was opened without delay on this occasion. Martha Dempster looked at him a moment, as if trying to place from where she knew him.

‘Dr Raven?’

Perhaps it was merely in contrast to Iain McKinnon’s understandably dour demeanour, but he thought she seemed pleased to see him.

‘Miss Dempster. Sorry to trouble you again, but I wondered whether on this occasion your sister might be home?’

‘No,’ she answered, Raven reading her countenance in vain for any hint that this might not be true. ‘But do come in out of the cold,’ she added.

She ushered him into a small drawing room at the front of the house, the door of which had been closed last time. Raven wondered what had been behind it on that occasion. The room looked expensively furnished, though many of the fixtures seemed overly grand and out of scale for the room. He had previously assumed that this had been the Dempster family’s home but now thought that he had been mistaken. The furniture must have belonged to her parents and been brought here from another house after they died.

‘Can I offer you some tea?’ she asked.

As long as your sister isn’t preparing it, Raven thought. How vulnerable we all were when we placed our trust in others, how exposed when others harboured evil intent. How much more so in the case of a nurse, when they were entrusted to administer medicines.

‘That would be most welcome,’ he answered.