The fog came in off the Thames like a living thing, thick and yellow and smelling of coal smoke and river mud. Thomas Blackwood pulled his coat tighter and leaned forward on the driver's seat of his hansom cab, watching the gas lamps gutter and flare in the wind. Three years it had been since Mary died, three years since the cholera took her in that small room above the bakery on Commercial Street, and still he drove these same streets, past the same doorways, past the same shadows that sometimes looked like her.

The clock in the cab showed half past eleven. He should have gone home hours ago. Emily would be asleep in her little bed, the one she had shared with her mother, and Mrs. Gable down the lane would be checking on her, as she did every night. Thomas trusted Mrs. Gable. He had to. There was nothing else for a man to do.

He turned onto Leadenhall Street and saw her then—a woman in a red dress, running through the fog, her heels clicking against the cobblestones. She was alone, and behind her, emerging from the mist like figures from a nightmare, came two men in dark coats.

Thomas felt something move in his chest, something he had thought dead with Mary. He cracked the whip without thinking, and the horse broke into a trot, then a canter, closing the distance between him and the woman. She was perhaps twenty-two, her face pale and streaked with tears, her breath coming in ragged gasps.

"Help!" she cried when she saw the cab, and Thomas felt the word settle into his bones like a command he could not refuse.

He drew rein beside her. "Get in," he said, and she did, stumbling into the cab and pulling the door shut behind her. The two men were close now, their faces obscured by the fog, but Thomas could see enough—the set of their shoulders, the purpose in their stride.

He whipped the horse again, and they lurched forward into the fog. Behind them, the two men cursed and broke into a run, but the fog swallowed them as quickly as it had swallowed the woman.

Thomas drove without knowing where he was going. The woman sat huddled in the corner of the cab, her red dress soaked with rain and fog, her hands clasped together like a prayer. He could not bring himself to look at her directly.

"Thank you," she whispered after a long silence. "Thank you."

He nodded. "Where to?"

She hesitated. "Anywhere. Just—keep driving."

He drove. The fog was so thick he could barely see the horse's ears, and the gas lamps were nothing more than yellow smudges in the mist. He thought of Mary, of the way she used to hum when she baked, of the little room above the bakery that now belonged to someone else.

After twenty minutes, he pulled up outside a newsstand on Fleet Street. An old man with milky eyes sat behind a stack of newspapers, his hands wrapped around a mug of tea.

"Mr. Peabody," Thomas said. "I need to borrow a match."

The old man turned his head slowly. He could not see, but he understood. Thomas had seen him do it before—other drivers, in other nights, tapping their whistles or asking for light, and Mr. Peabody would nod and reach into his pocket and produce a box of matches, and Thomas would take one, strike it, and drive on. It was a signal, though Thomas had never asked what it meant. He only knew that when Mr. Peabody gave you a match, help was on the way.

Mr. Peabody handed him a match. Thomas struck it against the side of the box, held it to his cigar, and turned back to the cab.

Two policemen emerged from the fog like ghosts, their helmets gleaming in the gaslight.

The woman's face went white. "No," she said. "Please, no."

Thomas frowned. "You're safe now."

"I'm not—" She stopped herself, swallowed hard, and looked away.

One of the policemen opened the cab door. "Miss, we're going to need you to come with us."

She looked at Thomas then, and in her eyes he saw something he could not name—not fear, not gratitude, but something darker and more complicated. Then she was gone, and the policemen were gone, and the fog was swallowing everything again.

Thomas drove home in silence. He told himself he had done the right thing. He told himself this every night for the three weeks that followed.

On the twenty-second night, he saw the body in the river.

It was early morning, the fog still thick, and he was driving past Blackfriars Bridge when a fisherman pointed and shouted. Thomas pulled over and looked down at the water, where the Thames carried something red and still toward the sea.

He knew before he went down. He knew the shape of the dress, the color of the hair, the way the water clung to fabric like a lover. When they pulled her up, she was face down, her hands bound behind her back with rope, and in her right hand, clenched so tightly they had to cut off the fingers to open it, was a small leather-bound book.

Thomas took the book. He could not explain why. It felt like a duty, like a debt he owed to a woman whose name he did not know.

He took it to a young detective at Scotland Yard named Croft, who was all sharp angles and restless energy and had not yet learned to look the other way. Thomas showed him the book, and Croft's face changed when he opened it—pages filled with names, dates, sums of money, and the seals of men whose names appeared on billboards and church foundations and the facades of grand buildings across London.

"This is—" Croft began, then stopped, closed the book, and looked at Thomas with something that might have been respect. "Where did you get this?"

Thomas told him. He told him about the woman in the red dress, the two men in the fog, the cab, the match, the policemen. He told him everything, and Croft listened without interrupting, and when Thomas finished, Croft said only: "You have no idea what you've done."

Thomas did not. Not then. He knew only that the woman was dead, and that he had tried to save her, and that somehow—somehow—the attempt had mattered, even if it had not mattered enough.

Croft took the book and disappeared into the fog of the Yard, and Thomas drove back into the fog of the streets, and for the first time in three years, he did not think of Mary.

In the weeks that followed, the names in the book began to fall. Men who had been untouchable found themselves investigated, indicted, ruined. The papers called it the greatest scandal in living memory. Thomas read the articles in his small room above the stable, and he felt nothing—no satisfaction, no pride, only a hollow ache where something had been, and was no longer.

He began to notice other drivers using the signal. A tap on the window. A question about a match. A nod toward Mr. Peabody's stand. They did not speak of what it meant. They did not need to.

One night, a young driver pulled up beside him at a red light and said, quietly: "Mr. Peabody gave me a match last week. Three men were dragging a woman into a van."

Thomas nodded.

"What happened?" the young driver asked.

"They're in custody," Thomas said.

The young driver looked at him with something like awe. "You're the one who started it, aren't you?"

Thomas did not answer. He looked at the fog rolling in off the Thames, yellow and thick and alive, and he thought of the woman in the red dress, and he thought of Mary, and he thought of all the women who would never be found, and he thought of all the matches that would be struck in the dark, and he said nothing at all.

He drove on.