“It’s beautiful,” Zanna said.

The girls held on to the pole and leaned out over the roofs.

“God,” said Zanna. “My dad would be sick if he saw me doing this.”

“Eeurgh,” said Deeba. “Imagine.” She leaned over and made a puking noise. “It’d go everywhere. ”

Conductor Jones stood on the platform with them, and they both knew somehow that if they were to slip, he’d be there to grab them.

The bus puttered low over the streets. Towers poked up around it. UnLondoners looked up and waved at it.

They passed squat tower blocks, arches of brick and stone, the hotchpotch slopes of roofs. There were stranger things, too: skyscraper-high chests of drawers in polished wood, spires like melting candles, houses like enormous hats and bats. Deeba pointed at gargoyles and pigeons on some of the houses, then started: some of the gargoyles were moving.

“Your eyes,” said Jones. “Bigger than fried eggs. I remember seeing it the first time.”

He pointed out landmarks to them.

“That’s Wraithtown, where the roofs flicker. That’s the market. Those windowless towers? Backwall Maze. That big fat chimney-thing? It’s the entrance to the library.”

“Why you here?” Zanna said.

“I couldn’t do this back in London, could I?” Jones held on to the pole with one hand and leaned out over the city. “Do you see that?” He pointed at a building made from typewriters and dead televisions.

“We saw one like that before,” Zanna said. “Obaday called it…what was it?”

“A moily house?” said Deeba.

“You’ll see a lot of moil technology here,” Jones said. “Em Oh Aye Ell. Mildly Obsolete In London. Throw something away and you declare it obsolete. You’ve seen an old computer, or a broken radio or whatever, left on the streets? It’s there for a few days, and then it’s just gone.

“Sometimes rubbish collectors have taken it, but often as not it ends up here, where people find other uses for it. It seeps into UnLondon. You might see residue: maybe a dried-up puddle on a wall. That’s where moil dripped through. And here, it sprouts like mushrooms on the streets.

“The money your friend has? All the out-of-date and foreign coins and notes Londoners throw away. A few years ago when Europe got rid of its old money and you were all left with loads of useless old bits and bobs, so much found its way down here we had too much, and that meant terrible inflation. We had to feed loads of it to the moolaphage…Anyway. That’s sort of how things get down here.

“You could say I was a bit like that,” he said thoughtfully. “Obsolete, they said. If you find just the right manhole you can get here. The hard part wasn’t coming through, it was getting the bus through.

“I always worked on the buses, back in London. You probably grown up paying the driver, right? Or travelcards? Didn’t used to be that way. It used to be that most of the buses in London had a driver and conductor.

“I’d take the money and give the tickets.” He patted the machine he wore. “It was quicker, because the driver didn’t have to deal with everyone. And it was safer. Two of us there, all the time. But they decided they could save money if they got rid of half of us. Of course it messed things up. But them who made the decision were people who never took buses, so they didn’t care.

“We knew what we did was important. Look in the dictionary. ‘Conduct: verb. To lead, control, or guide. ’ Some of us weren’t prepared to stop being guides. We look after travelers. It’s…” Conductor Jones looked down, suddenly shy. “Some people say it’s a sacred duty.”

“UnLondon…Well, sometimes, it can be a dangerous place. We had to be really ready to conduct. ” He tapped the weapon on his belt, pointed into the cabinet beside him, at a bow and arrow, and coils of wire. “The drivers who came down swore to get the passengers from where they are to where they want to go. And to protect them.”

“Protect them from what?” Zanna said.

“There’s the occasional skymugger,” Jones said. “Airsquid, though mostly they hunt high, where the deep-sky fisherfolk go. And there are other things. Conductors on other routes, if they’re real unlucky, sometimes get attacked by giraffes.”

The girls stared at each other.

“You’re the second person to say that,” Deeba said.

“I’ve seen giraffes,” Zanna said.

“They’re so not scary…” Deeba said.

“Ha!” The whole bus looked up at Jones’s laugh. “They’ve done a good job making people believe that those hippy refugees in the zoo are normal giraffes. Next you’ll tell me that they’ve got long necks so they can reach high leaves! Nothing to do with waving the bloody skins of their victims like flags, of course.

“There’s a lot of animals very good at that sort of disinformation. There are no cats in UnLondon, for example, because they’re not magic and mysterious at all, they’re idiots. You’ll find pigs, dogs, frogs, everything else getting through to here, though. There’s a lot of traffic back and forth. They know when things are happening. They pass messages.”

“Zann,” said Deeba. “That makes sense. All those animals, they knew you were…whatever you are.”

“The Shwazzy,” said Zanna.

“But no cats,” Jones went on. “Too busy trying to look cool. Anyway. You know what the main danger is down here. And it’s a danger that’s been growing. For years.”

“The Smo—” Zanna said, and he put his finger quickly to his lips.

“Yes,” he said. “That’s why you’re here.”

“But what is it?” she said. “What does it want?”

“Can’t talk about it here,” Jones whispered. “Better safe than. You know what I mean. The Propheseers’ll explain.

“Next stop,” he yelled. “Manifest Station.”

They passed an enormous building like a cathedral, within a few meters of its windows, and people peered at them out of offices. The edifice was perforated in several places with what looked like random holes, and bursting from them were railway lines. They sprang in different directions: horizontal; up like a roller coaster; corkscrewing down. A few hundred meters from the great building, they plunged into holes in the street, and down into darkness.

“Manifest Station,” Jones said. A dark-windowed diesel train burst out of the building, close enough to make the bus shake. It helter-skeltered downward into the earth.

“Where’s it going?” Zanna said.

“Crossing the Odd, to some of the other abcities,” Jones said. “If you’re brave enough to try, you might be able to catch a train from UnLondon to Parisn’t, or No York, or Helsunki, or Lost Angeles, or Sans Francisco, or Hong Gone, or Romeless…It’s a terminus.”

They hovered above a big yard at the side of the station containing twenty or thirty double-decker buses, with passengers milling around them. Each bus had a different sign where the numbers should be— faces, insects, flowers, random patterns. On their sides, where London buses carried adverts, were paintings, short stories in big print, pictures of chessboards with games in progress, musical scores.

But these were details. What made Zanna and Deeba stare and make little sounds of wonder was how the buses moved.

UnLondon’s terrain was difficult. There were thin tangled streets, sudden steep hills, deep pits, patches where roads seemed to be made of something too soft for wheels, on which pedestrians bounced. To deal with the various difficulties of their routes, the UnLondon buses had adapted.

They trundled on caterpillar treads. They rolled on enormously inflated rubbery wheels. They coasted on skirts of air like hovercrafts. In the sky was another aerobus, below a round balloon. Conductors leaned out of the vehicles, bristling with weapons.

A bus approached the terminus from a thicket of tall spindly towers. It picked its way over the roofs on four enormous lizard legs that sprouted from its wheel housings. The driver spun the wheel and tugged at levers, and the bus’s padded gecko feet closed gently around buttresses and splayed on slanting roofs, leaving no marks behind.

“Manifest Station Terminus,” Jones belted. “Who’s changing here?”

They winched Mrs. Jujube and two other passengers down in a basket. “This is the Scrollscrawl bus, and you want the Rusty Star Sigil bus,” Jones told one. “And you, sir, look for the Terrible Mouse Sigil.”

As the bus swung in position, Deeba looked up and made a little startled noise.

“What is it?” said Zanna.

“I thought I saw something,” said Deeba, pointing up. “Like…a crab. Moving on the ceiling.”

“Well…” Zanna looked around. “It’s gone now. This place is full of weird things.”

The basket dangled between a bus on stilts and another on what looked like giant ice skates. The three passengers got out. At the last moment a man wearing a toga ran and caught the basket; said good-bye to a companion, who hurried off; and got in.

He was big and heavy to haul. When he stepped onto the platform, there was a hissing. The milk carton was huddled behind Deeba, exhaling aggressively.

“Curdle,” Zanna muttered. “Deeba, keep your manky[12] pet under control.”

The new passenger stared huffily at Curdle from behind his beard.

“See that?” Deeba whispered.

“That bloke does not like us.”

They went much higher this time, midway between the roofs and the strangely lit sky. UnLondon sprawled to the horizon. A few animal-footed buses crawled carefully over and around houses. Light from the empty sun gleamed on a million surfaces. It was a ragged and jagged landscape. Low clouds buzzed below their wheels, obscuring neighborhoods, moving in various directions purposefully.

“That?” Jones pointed at what looked like a shirt, racing madly through the air. “When washing blows away in London, if it stays in the air long enough, it blows all the way here. Then it’s free. Never has to come down.”

They passed a stepped pyramid, a corkscrew-shaped minaret, a building like an enormous U.

“I wish my mum was here,” Deeba whispered. She couldn’t even look up as the thought took her. “And my dad. Even my brother Hass.”

“Me too,” said Zanna.