“To scare people, or maybe to remind them of their duty,” Barnabus thought out loud. “It was certainly etched in Serena’s mind as she stood there. We may have stumbled across not merely a randomly scrawled cryptic message but a hideout or gathering place connected with the Mastiff and his people.”

“Could be. And Shoreditch’d be the place for it.”

“Considering all the fires, writing the message in ash doesn’t seem accidental.”

“Ain’t terribly bright, either,” Brogan said. “At the first rainfall, all that’ll be left is a dingy puddle on the ground.”

The Mastiff, or whoever wrote it on his behalf, must have been certain the words would be seen quickly by whomever it was meant for. More evidence that the crumbling walls and dark alleys were a location of some significance.

Another knock echoed off the front door.

“Have you been busy all day, Doc?” Brogan asked.

“Yes, but not with patients. It’s been an odd day.”

“Well, offer my apologies to Gemma, but I’ll slip out the back door while you’re seeing to whoever’s at the front. It’s a fair jaunt back to Piccadilly.”

Barnabus walked from the library to the front door. He didn’t usually have so many visitors on the nights his surgery was closed.

A man stood on the front stoop, his hat in his hands, looking a bit upended. He was dressed nicely if not elegantly. He might have worked as a clerk or a merchant. Nothing was obviously the matter with him.

“May I help you?” Barnabus asked.

The man’s gaze narrowed, not ominously but in confusion. Something about his expression made him look familiar.

“I’m feeling a touch poorly,” he said. “Was hoping you could take a minute and let me know if it’s anything I ought to be worried about.”

Barnabus motioned him inside and directed him to the sitting room where he saw his patients. As the man passed him, Barnabus spotted a folded issue of the London Times tucked under his arm. That one clue pieced together the rest of the mystery. This was the man who’d watched him on the street earlier that day, the man who’d sat with a paper on his lap.

“I’m Dr. Barnabus Milligan,” he said. “What’s your name?”

“George Snelling.”

“Mr. Snelling, what is it that ails you?”

“I—There is . . . I . . .”

There was no mistaking he was trying to think of something. Most patients could tell him quickly, if in vague terms, what they were experiencing. Those who were afflicted with something potentially embarrassing would generally start by identifying it as such. This hesitation was neither of those things. And through it all the man kept watching him, studying him.

“My throat,” Mr. Snelling finally said. “I fear it might be the beginnings of a putrefaction.”