The sergeant took a step forward, and in that instant Caleb Stone saw Monk behind him. Perhaps it was some look of victory in Monk's face, maybe he recognized the sergeant. He climbed to his feet and lunged towards the win- dow, throwing himself out of it with a shattering of glass.

The fat man rolled over onto all fours and charged at Monk. Monk raised his knee and caught him in the jaw, sending him reeling backwards, spitting blood. The other man was locked in a struggle with the sergeant, swinging backwards and forwards together like a parody of a dance.

Monk ran over to the window and smashed the rest of the glass out of the frame, then leaned out, half expecting to see the figure of Caleb broken on the pavement four stories below.

But he had forgotten the twists and turns of the stairs. They were facing the back of the building, and beneath him was the roof of a high wooden shed, not more than twelve feet away. Caleb was running across it, agile as an animal, making for the opposite side and a half-open window.

Monk scrambled over the sill and leaped, landing with a jar that shocked his bones. Within a moment he was on his feet and racing after Caleb, the shed roof rattling under his weight.

Caleb swung around once, his wide mouth grinning, then he jumped for the window and disappeared inside.

Monk went in after him, finding himself in another cold, suffocating room just like those he had left. Three old men sat with bottles in their hands around a potbellied stove smelling of soot.

Caleb flung the door open and charged across the landing and Monk heard his footsteps hard on the stairs. He dived after him, tripped on the fourth or fifth step, which was broken, and fell the remaining half dozen, landing bruisingly and only just missing cracking his head on the newel post. He heard Caleb's laughter as he clattered on down, a floor below him. Monk clambered to his feet, furious with pain and frustration, and went down the rest of the stairs as fast as he could. He was just in time to see Caleb's back as he went out the door into Prestage Street and turned towards Brunswick Street, which ran all the way down to the river, Ashton's Wharf and the Blackwall Stairs.

Where the devil were the other constables? Monk yelled as loudly as his lungs would bear.

“Benyon! Brunswick Street!”