The fog rolled through the streets of London like a living thing, thick and yellow with the exhalations of a million factory chimneys. Thomas Harrington woke in an alley off Wapping High Street and knew, with a certainty that bypassed thought, that he was no longer quite human.

It had happened three nights ago at the chemical works on the Thames waterfront. A valve had burst—nobody knew whose fault it was, and nobody would ever know, because the foreman had locked the doors when the shift ended and the fire would consume every witness including the evidence. Thomas had been sweeping floors. Nineteen years old, orphaned, the sort of boy the city produced by the thousands and digested without noticing.

When he opened his eyes in the alley, the first thing he noticed was the smell. Not the usual London cocktail of coal smoke and river rot and something unnameable that was probably human. This was different. Beneath the stench, he could detect individual molecules—hydrogen sulfide from the tanneries upstream, ammonia from the gasworks, the faint metallic tang of the river itself. His nose had become an instrument of impossible precision.

Then came the touching of his skin. His arms felt wrong. Soft. Damp. He pulled back his sleeve and screamed, but the scream came out wrong too—thin and reedy, like wind through a cracked pipe.

His skin had gone pale and translucent. Tiny bristles, no thicker than hair, covered his forearms. When he pressed his fingers against the cobblestones, he could feel every individual stone, every crack, every grain of sand packed between them. The texture of the world had become legible to his fingertips like a blind man reading.

Mary found him at dawn. She was a girl from the next alley over, seventeen if she was a day, blind since the fever took her eyes two years before. She moved through the dark streets with the confidence of someone who had never needed to see to find her way.

"Thomas?" she said, her head tilted like a bird listening for worms. "Is that you? You smell different."

He wanted to tell her. He wanted to run to her and hold on to the one human thing left in a world that had suddenly become alien. But when he opened his mouth, all that came out was a wet, clicking sound.

Mary reached out and found his arm. Her fingers traced the bristles, and she frowned. "You're covered in little hairs. What happened to you, Tom?"

He took her hand and pressed it against his chest. His heart was beating faster than it should have—slower, but stronger, each beat sending a pulse of blood through his body like a wave through water. He could feel the river beneath their feet, the underground streams that ran beneath all of London, the vast dark network of tunnels and pipes and forgotten places where the city's waste went to disappear.

Mary listened to his heart and nodded slowly. "I can hear it. It's very loud. Like the tide."