In chasing his new disease, Raven had seen only the evidence that suited him, only the evidence he had wanted to see. In that room, he had seen only the evidence someone else wanted him to see. It was a tableau, though not one intended to be presented to him, but to Martha.

Was it possible someone else sought to pin the blame for their deeds on Mary Dempster, leaving her poisoned body and her confession to be found by her sister? If so, how could the confession be in her words, her hand? Yet it had to be, he reasoned, for if it was not, Martha would have been able to tell instantly, and she was the one who had been summoned. So, if Mary had truly written her confession and killed herself, who had been in that room with her, sharing a bottle of gin?

He pictured the corpse, its face eaten away by rats, only her nurse’s dress to identify her. Raven had once learned in the most painful manner not to make assumptions about who was inside a set of clothes.

He belatedly saw the truth of it. The other woman in that room had not been Mary Dempster’s killer, but her latest victim: some prostitute of the West Port, lured there and poisoned, dressed in Mary’s clothes and left alongside Mary’s confession. A substitute left in her place so that Mary could abscond and those who had discovered her crimes would assume the author of them dead.

If that were so, he reasoned, she would have to move on, assume a new identity.

Then it dawned on him that she already had.

The Night Asylum fell silent, the noise of the other prisoners fading into the background until Raven was the only person in there, briefly alone with the most terrible discovery.

He knew the answer to the question of why Mary spared Martha.

She hadn’t.

That nurse’s dress: dark blue with braiding on the cuffs. He had seen it in Mary’s bedroom at the cottage. Martha had been anxious when she found him looking in there, even though her sister was not home. Why, he had wondered, would Mary have the grand master bedroom, when it was Martha’s inheritance that paid for them to stay there?

Because only one person lived in that house.

Martha’s words echoed in his head: It amused us down the years how often people would say we had a resemblance while they thought we were true sisters. Introduce an idea into someone’s head and they will soon start to paint a picture around it.

She knew Raven had seen her as Mary, albeit briefly, and had suggested a plausible explanation should he notice a similarity.