Raven was about to decline but the door was opened wide and he was ushered inside. He didn’t particularly want to speak to Sarah’s husband and certainly not without her present to moderate.

He was shown into the parlour, where Archie was already seated in an armchair by the fire.

‘Do sit down, Will.’

He seemed open and friendly, as though they had known each other for some time. Raven was wary. He perched himself on the edge of a chair, hoping to give the impression that he was not at liberty to stay for any length of time.

He looked at the man sitting opposite: really looked, searching for confirmation of his suspicions. Archie was indeed thin, perhaps thinner than the last time Raven had seen him, but he couldn’t be sure. He was pale but not uncommonly so. Raven had to admit that this was hardly conclusive evidence, but he felt intuitively that this man was far from well. Or did he merely wish that it was so?

‘I am so glad that you called by,’ Archie said. ‘We haven’t really had much of a chance to talk.’

Raven had been hoping that this was a state of affairs that might long endure. His instinct was to imagine that there was nothing they had in common to talk about, but if he were being honest, his reluctance derived from the opposite being true. They were both men of medicine, for one thing. But primarily the thing – or rather the person – they had in common was now Archie’s wife.

‘Sarah has told me so much about you,’ he said cheerily. ‘It seems you two had quite the adventure together, once upon a time. A secret alliance engaged in subterfuge and hazardous enterprises.’

Raven swallowed, wondering just how much Sarah had told Archie about their time together. Had it been anyone else, he could have been assured that common discretion protected his confidence, but this was Sarah. Little about her was common, and that included her relationship with her new husband.

‘How did you two encounter each other?’ Raven asked, eager to shift the focus of discussion. He was also impatiently curious.

‘I was visiting the professor,’ Archie said. ‘I first crossed paths with James when we were both students here in Edinburgh. He was several years my senior, but we got on well and have kept up a regular correspondence since.’

Raven wondered if the visit being alluded to was as a colleague or as a patient but felt it would be impertinent to ask.

‘When I arrived here from Perth in the summer, I left my card at Queen Street and then took a room at the Royal Hotel. I had only just begun to unpack my portmanteau when Dr Simpson appeared at my door in the company of Mr Gibb, the proprietor. He said that I must come and stay at Queen Street as his guest.