Never give the quarry time to rest. He'd learned that on his first day in the Watch. If you had to chase, then stay with it. Give the pursued time to stop and think and you'd go round a corner to find a sock full of sand coming the other way.

The walls and ceiling were closing in.

There were other tunnels here. Carrot had been right. Hundreds of people must have worked for years to build this. What Ankh-Morpork was built on was Ankh-Morpork.

Vimes stopped.

There was no sound of splashing, and tunnel mouths all around.

Then there was a flash of light, up a side tunnel.

Vimes scrambled towards it, and saw a pair of legs in a shaft of light from an open trapdoor.

He launched himself at them, and caught a boot just as it was disappearing into the room above. It kicked at him, and he heard Cruces hit the floor.

Vimes grabbed the edge of the hatchway and struggled through it.

This wasn't a tunnel. It looked like a cellar. He slipped on mud and hit a wall clammy with slime. What was Ankh-Morpork built on? Right…

Cruces was only a few yards away, scrambling and slipping up a flight of steps. There had been a door at the top but it had long ago rotted.

There were more steps, and more rooms. Fire and flood, flood and rebuilding. Rooms had become cellars, cellars had become foundations. It wasn't an elegant pursuit; both men slithered and fell, clambered up again, fought their way through hanging curtains of slime. Cruces had left candles here and there. They gave just enough light to make Vimes wish they didn't.

And then there was dry stone underfoot and this wasn't a door, but a hole knocked through a wall. And there were barrels, and sticks of furniture, ancient stuff that had been locked up and forgotten.

Cruces was lying a few feet away, fighting for breath and hammering another rack of pipes into the gonne. Vimes managed to pull himself up on to his hands and knees, and gulped air. There was a candle wedged into the wall nearby.

“Got… you,” he panted.

Cruces tried to get to his feet, still clutching the gonne.

“You're… too old… to run…” Vimes managed.

Cruces made it up upright, and lurched away. Vimes thought about it. “I'm too old to run,” he added, and leapt.

The two men rolled in the dust, the gonne between them. It struck Vimes much later that the last thing any man of sense would do was fight an Assassin. They had concealed weapons everywhere. But Cruces wasn't going to let go of the gonne. He held it grimly in both hands, trying to hit Vimes with the barrel or the butt.

Curiously enough, Assassins learned hardly any unarmed combat. They were generally good enough at armed combat not to need it. Gentlemen bore arms; only the lower classes used their hands.

“I've got you,” Vimes panted. “You're under arrest. Be under arrest, will you?”

But Cruces wouldn't let go. Vimes didn't dare let go; the gonne would be twisted out of his grip. It was pulled backwards and forwards between them in desperate, grunting concentration.

The gonne exploded.

There was a tongue of red fire, a firework stink and a zing-zing noise from three walls. Something struck Vimes' helmet and zinged away towards the ceiling.

Vimes stared at Cruces' contorted features. Then he lowered his head and yanked the gonne hard.

The Assassin screamed and let go, clutching at his nose. Vimes rolled back, gonne in both hands.

It moved. Suddenly the stock was against his shoulder and his finger was on the trigger.

You're mine.

We don't need him any more.

The shock of the voice was so great that he cried out.

He swore afterwards that he didn't pull the trigger. It moved of its own accord, pulling his finger with it. The gonne slammed into his shoulder and a six-inch hole appeared in the wall by the Assassin's head, spraying him with plaster.

Vimes was vaguely aware, through the red mist rising around his vision, of Cruces staggering to a door and lurching through it, slamming it behind him.

All that you hate, all that is wrong—I can put it right.

Vimes reached the door, and tried the handle. It was locked.

He brought the gonne around, not aware of thinking, and let the trigger pull his finger again. A large area of the door and frame became a splinter-bordered hole.

Vimes kicked the rest of it away and followed the gonne.

He was in a passageway. A dozen young men were looking at him in astonishment from half-open doors. They were all wearing black.

He was inside the Assassins' Guild.

A trainee Assassin looked at Vimes with his nostrils.

“Who are you, pray?”

The gonne swung towards him. Vimes managed to haul the barrel upwards just as it fired, and the shot took away a lot of ceiling.

“The law, you sons of bitches!” he shouted.

They stared at him.

Shoot them all. Clean up the world.

“Shut up!” Vimes, a red-eyed, dust-coated, slime-dripping thing from out of the earth, glared at the quaking student.

“Where did Cruces go?” The mist rolled around his head. His hand creaked with the effort of not firing.

The young man jerked a finger urgently towards a flight of stairs. He'd been standing very close when the gonne fired. Plaster dust draped him like devil's dandruff.

The gonne sped away again, dragging Vimes past the boys and up the stairs, where black mud still trailed. There was another corridor there. Doors were opening. Doors closed again after the gonne fired again, smashing a chandelier.

The corridor gave out on to a wide landing at the top of a much more impressive flight of stairs and, opposite, a big oaken door.

Vimes shot the lock off, kicked at the door and then fought the gonne long enough to duck. A crossbow bolt whirred over his head and hit someone, far down the corridor.

Shoot him! SHOOT HIM!

Cruces was standing by his desk, feverishly trying to slot another bolt into his bow—

Vimes tried to silence the singing in his ears.

But… why not? Why not fire? Who was this man? He'd always wanted to make the city a cleaner place, and he might as well start here. And then people would find out what the law was…

Clean up the world.

Noon started.

The cracked bronze bell in the Teachers' Guild began the chime, and had midday all to itself for at least seven clangs before the Guild of Bakers' clock, running fast, caught up with it.

Cruces straightened up, and began to edge towards the cover of one of the stone pillars.

“You can't shoot me,” he said, watching the gonne. “I know the law. And so do you. You're a guard. You can't shoot me in cold blood.”

Vimes squinted along the barrel.

It'd be so easy. The trigger tugged at his finger.

A third bell began chiming.

“You can't just kill me. That's the law. And you're a guard,” Dr Cruces repeated. He licked his dry lips.

The barrel lowered a little. Cruces almost relaxed.

“Yes. I am a guard.”

The barrel rose again, pointed at Cruces' forehead.

“But when the bells stop,” said Vimes, quietly, “I won't be a guard any more.”

Shoot him! SHOOT HIM!

Vimes forced the butt under his arm, so that he had one hand free.

“We'll do it by the rules,” he said. “By the rules. Got to do it by the rules.”

Without looking down, he tugged his badge off the remains of his jacket. Even through the mud, it still had a gleam. He'd always kept it polished. When he spun it once or twice, like a coin, the copper caught the light.

Cruces watched it like a cat.

The bells were slackening. Most of the towers had stopped. Now there was only the sound of the gong on the Temple of Small Gods, and the bells of the Assassins' Guild, which were always fashionably late.