The factory whistle screamed at half past six, but Thomas Blackwood did not hear it. He heard only the grinding of iron in his skull, the slow calcification of his own eyes, and the shimmering outlines of everything that would be lost.

It began on the morning of the accident. Thomas had been twenty-four, a mill apprentice in Whitechapel, and he had been pulling a jammed gear when the belt snapped. The iron wheel caught him by the chest and threw him against the brick wall with enough force to crack three ribs and stop his heart for four minutes. When he woke in the infirmary, the world had changed.

Not the world itself. The world was still the same soot-stained London he had always known. But Thomas could now see things that no one else could see. Shimmering outlines, like heat haze, hovering over every person he looked at. Each outline showed what that person was about to lose.

At first, Thomas thought it was a blessing. He saw the mill foreman's outline shimmering with the loss of his position — a promotion was coming, and the foreman would be replaced. Thomas warned him. The foreman laughed, then took the warning seriously, and survived the restructuring that swept through the mill three weeks later. Thomas saw a young washerwoman named Mary about to lose her position — the landlord was evicting everyone on her street. Thomas told her. She moved two days before the eviction notice arrived.

Each time Thomas used his gift, something left him. Not dramatically. Not all at once. A slow erosion, like water wearing stone. First he lost his position at the mill — not because he warned people, but because he was always late, always distracted, always looking at the shimmering outlines that only he could see. The foreman called him mad. Thomas did not argue.

Then he lost his friends. They stopped inviting him to the pub because Thomas would sit in the corner, staring at their outlines, saying nothing. They could feel his pity, and pity from a friend is worse than contempt. William, his closest friend since childhood, stopped speaking to him after Thomas told him that he would lose his daughter to fever. William called him a curse. Thomas did not correct him.

Martha was the one who stayed. She was twenty-two, with hair the color of dark bread and hands that could stitch a wound without flinching. She worked at the hospital on Commercial Street, and she was the only person whose outline Thomas could not see. When he looked at her, he saw nothing. No shimmering. No loss. Just Martha, with her steady eyes and her quiet smile.

For three months, Thomas allowed himself to believe that Martha was different. That she was safe.

Then the outline appeared.

It was a Tuesday in November, and Thomas had been walking Martha home from the chapel. The fog was thick, the kind of Whitechapel fog that tastes of coal and decay. He was looking at her — really looking at her, the way he had been looking at her for three months — when he saw it. A faint shimmer, barely visible, hovering just above her shoulder. The outline of something she was about to lose.

Thomas stopped walking. His heart hammered against his ribs like a trapped bird. He stared at the shimmer, trying to understand what it meant. What could he lose? What could she lose?

"Thomas?" Martha stopped and turned to him. "What is it?"

"Nothing," he said. "Just tired."

But he was not tired. He was terrified.

Over the next weeks, Thomas watched Martha's outline every day. The shimmer grew stronger, more defined. It was the outline of death. Not a metaphor. Not a loss of position or health or love. Death. The actual, concrete, irreversible end of Martha Blackwood.

Thomas tried everything. He kept her home from the hospital. He locked the doors and barred the windows. He brought her food and water and sat by her bed like a guard. He told himself that if he could just prevent whatever was coming, the outline would disappear.

It did not disappear. It grew brighter.

On the thirty-seventh night, Thomas made his final attempt. He told Martha he was going to the mill to retrieve something — a locket, he said, that she had left behind. In truth, he was going to the Thames. He had read in the papers that the water was rising, that the fog had made the banks treacherous. If he could keep her away from the river, away from everything, perhaps he could save her.

He came home at midnight. The house was dark. Martha was not in bed.

Thomas found her on the roof. She was standing at the edge, looking out over Whitechapel, at the sea of fog and gaslight and iron rooftops that stretched to the horizon. She turned when he approached, and she smiled.

"You should not have come up here, Thomas," she said.

The edge gave way. It was not dramatic. There was no struggle, no cry. Just the sound of brick crumbling under her foot, and then the sound of her falling through the fog, fading into the darkness below.

Thomas stood at the edge of the roof and looked down. He could not see her. He could only see the shimmer, growing fainter and fainter, until it was gone.

After that, Thomas sat by the Thames every evening. The fog had turned his eyes — the whites were no longer white, but the dull gray of old iron. He could still see the outlines, but he no longer used them. He understood now. The Iron Gaze was not foresight. It was a curse. Every time he saw what someone would lose, he accelerated the loss. Every act of prevention was itself the cause.

He had tried thirty-seven times to save Martha. Each time, his prevention had been the very thing that killed her. The last time, he had gone to the roof to stop her from going out. His arrival had made her go to the edge. His love had been the weapon.

Thomas put his hand to his face and felt the cold iron of his own eyes. He thought of William, of Mary, of all the people he had tried to save. He thought of Martha, standing at the edge of the roof, smiling at him.

The Thames flowed on, dark and indifferent, carrying the rust and ash of London toward the sea.