'That's Detritus the troll, sir.'

'Why is he sitting like that?'

'He's thinking, sir.'

'He hasn't moved for some time.'

'He thinks slow, sir.'

Detritus stood up. There was something about the way he did it, some hint of a mighty continent beginning a tectonic movement that would end in the fearsome creation of some unscalable mountain range, which made people stop and look. Not one of the watchers was familiar with the experience of watching mountain building, but now they had some vague idea of what it was like: it was like Detritus standing up, with Cuddy's twisted axe in his hand.

'But deep, sometimes,' said Nobby, eyeing various possible escape routes.

The troll stared at the crowd as if wondering what they were doing there. Then, arms swinging, he began to walk forward.

'Acting-Constable Detritus . . . er . . . as you were Colon ventured.

Detritus ignored him. He was moving quite fast now, in the deceptive way that lava does.

He reached the wall, and punched it out of the way.

'Has anyone been giving him sulphur?' said Nobby.

Colon looked around at the guard. 'Lance-Constable Bauxite! Lance-Constable Coalface! Apprehend Acting-Constable Detritus!'

The two trolls looked first at the retreating form of Detritus, then at one another, and finally at Sergeant Colon.

Bauxite managed a salute.

'Permission for leave to attend grandmother's funeral, sir?'

'Why?'

'It her or me, sarge.'

'We get our goohuloog heads kicked in,' said Coalface, the less circuitous thinker.

A match flared. In the sewers, its light was like a nova.

Vimes lit first his cigar, and then a lamp.

'Dr Cruces?' he said.

The chief of Assassins froze.

'Corporal Carrot here has a crossbow too,' he said. 'I'm not sure if he'd use it. He's a good man. He thinks everyone else is a good man. I'm not. I'm mean, nasty and tired. And now, doctor, you've had time to think, you're an intelligent man . . . What were you doing down here, please? It can't be to look for the mortal remains of young Edward, because our Corporal Nobbs has taken him off to the Watch morgue this morning, probably nicking any small items of personal jewellery he had on him, but that's just Nobby's way. He's got a criminal mind, has our Nobby. But I'll say this for him: he hasn't got a criminal soul.

'I hope he's cleaned the clown make-up off the poor chap. Dear me. You used him, didn't you? He killed poor old Beano, and then he got the gonne, and he was there when it killed Hammerhock, he even left a bit of his Beano wig in the timbers, and just when he could have done with some good advice, such as to turn himself in, you killed him. The point, the interesting point, is that young Edward couldn't have been the man on the Tower a little while ago. Not with the stab wound in his heart and everything. I know that being dead isn't always a barrier to quiet enjoyment in this city, but I don't think young Edward has been up and about much. The piece of cloth was a nice touch. But, you know, I've never believed in that stuff – footprints in the flower bed, telltale buttons, stuff like that. People think that stuff's policing. It's not. Policing's luck and slog, most of the time. But lots of people'd believe it. I mean, he's been dead . . . what. . . not two days, and it's nice and cool down here . . . you could haul him up, I daresay you could fool people who didn't look too close once he was on a slab, and you'd have got the man who shot the Patrician. Mind you, half the city would be fighting the other half by then, I daresay. Some more deaths would be involved. I wonder if you'd care.' He paused. 'You still haven't said anything.'

'You have no understanding,' said Cruces.

'Yes?'

'D'Eath was right. He was mad, but he was right.'

'About what, Dr Cruces?' said Vimes.

And then the Assassin was gone, diving into a shadow.

'Oh, no,' said Vimes.

A whisper echoed around the man-made cavern.

'Captain Vimes? One thing a good Assassin learns is—'

There was a thunderous explosion, and the lamp disintegrated.

'—never stand near the light.'

Vimes hit the floor and rolled. Another shot hit a foot away, and he felt the splash of cold water.

There was water under him, too.

The Ankh was rising and, in accordance with laws older than those of the city, the water was finding its way back up the tunnels.

'Carrot,' Vimes whispered.

'Yes?' The voice came from somewhere in the pitch blackness to his right.

'I can't see a thing. I lost my night vision lighting that damn lamp.'

'I can feel water coming in.'

'We—' Vimes began, and stopped as he formed a mental picture of the hidden Cruces aiming at a patch of sound.

I should have shot him first, he thought. He's an Assassin!

He had to raise himself slightly to keep his face out of the rising water.

Then he heard a gentle splashing. Cruces was walking towards them.

There was a scratching noise, and then light. Cruces had lit a torch, and Vimes looked up to see the skinny shape in the glow. His other hand was steadying the gonne.

Something Vimes had learned as a young guard drifted up from memory. If you have to look along the shaft of an arrow from the wrong end, if a man has you entirely at his mercy, then hope like hell that man is an evil man. Because the evil like power, power over people, and they want to see you in fear. They want you to know you're going to die. So they'll talk. They'll gloat.

They'll watch you squirm. They'll put off the moment of murder like another man will put off a good cigar.

So hope like hell your captor is an evil man. A good man will kill you with hardly a word.

Then, to his everlasting horror, he heard Carrot stand up.

'Dr Cruces, I arrest you for the murder of Bjorn Hammerhock, Edward d'Eath, Beano the clown, Let-tice Knibbs and Acting-Constable Cuddy of the City Watch.'

'Dear me, all those? I'm afraid Edward killed Brother Beano. That was his own idea, the little fool. He said he hadn't meant to. And I understand that Hammerhock was killed accidentally. A freak accident. He poked around and the charge fired and the slug bounced off his anvil and killed him. That's what Edward said. He came to see me afterwards. He was very upset. Made a clean breast of the whole thing, you know. So I killed him. Well, what else could I do? He was quite mad. There's no dealing with that sort of person. May I suggest you step back, sire? I'd prefer not to shoot you. No! Not unless I have to!'

It seemed to Vimes that Cruces was arguing with himself. The gonne swung violently.

'He was babbling,' said Cruces. 'He said the gonne killed Hammerhock. I said, it was an accident? And he said no, no accident, the gonne killed Hammerhock.'

Carrot took another step forward. Cruces seemed to be in his own world now.

'No! The gonne killed the beggar girl, too. It wasn't me! Why should I do a thing like that?'

Cruces took a step back, but the gonne swung up towards Carrot. It looked to Vimes as though it moved of its own accord, like an animal sniffing the air . . .

'Get down!' Vimes hissed. He reached out and tried to find his crossbow.

'He said the gonne was jealous! Hammerhock would have made more gonnes! Stop where you are!'

Carrot took another step.

'I had to kill Edward! He was a romantic, he would have got it wrong! But Ankh-Morpork needs a king!'

The gun jerked and fired at the same moment as Carrot leapt sideways.

The tunnels were brilliant with smells, mostly the acrid yellows and earthy oranges of ancient drains. And there were hardly any air currents to disturb things; the line that was Cruces snaked through the heavy air. And there was the smell of the gonne, as vivid as a wound.

I smelled gonne in the Guild, she thought, just after Cruces walked past. And Gaspode said that was all right, because the gonne had been in the Guild – but it hadn't been fired in the Guild. I smelled it because someone there had fired the thing.

She splashed through the water into the big cavern and saw, with her nose, the three of them – the indistinct figure that smelled of Vimes, the falling figure that was Carrot, the turning shape with the gonne . . .

And then she stopped thinking with her head and let her body take over. Wolf muscle drove her forward and up into a leap, water droplets flying from her mane, her eyes fixed on Cruces' neck.

The gonne fired, four times. It didn't miss once.

She hit the man heavily, knocking him backwards.

Vimes rose in an explosion of spray.

'Six shots! That's six shots, you bastard! I've got you now!'

Cruces turned as Vimes waded towards him, and scurried towards a tunnel, throwing up more spray.

Vimes snatched the bow from Carrot, aimed desperately and pulled the trigger. Nothing happened.

'Carrot! You idiot! You never cocked the damn thing!'

Vimes turned.

'Come on, man! We can't let him get away!'

'It's Angua, captain.'

'What?'

'She's dead!'

'Carrot! Listen. Can you find the way out in this stuff? No! So come with me!'

'I . . . can't leave her here. I—'

'Corporal Carrot! Follow me!'

Vimes half ran, half waded through the rising water towards the tunnel that had swallowed Cruces. It was up a slope; he could feel the water dropping as he ran.

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.