“Come on,” said Zanna eventually. “We can’t just sit and feel sorry for ourselves.”

“I bet I could,” Deeba said, but she stood, holding Curdle.

“We deserve to,” Zanna said. “We just can’t. ”

The UnSun was getting lower, and the sky darker.

“We have to find somewhere to shelter,” Zanna said.

“And food,” said Deeba.

They clambered laboriously up the slope, hauled themselves onto the ridge, and stared.

They were in the middle of undulating roofs, a slatescape in red and gray and the color of rust. It rose and fell like mountainside, steep, shallow, deep, flat, interrupted by trenches where streets must run, unlit alleys between houses. The angles were broken by dormer windows, by squat chimneys like patches of mushrooms, by tangles of antennae, wire fingers pointing in all directions.

They stared for a long time in the direction Zanna thought she had seen something fall. They could see nothing moving over the up-and-down of the tiles.

“What do we do?” said Deeba. “How do we get anywhere?”

“I don’t know,” said Zanna. “Let’s try this…” She began to shuffle along the ridge. Deeba stared.

“You’re kidding,” she said. Sighing, she put Curdle in her bag, and— slowly— followed her friend.

They stopped, suddenly, as an awful bleating cry sounded nearby, and was answered from a long way off.

“What was that?” whispered Deeba.

“How should I know?” Zanna whispered back.

“Well I’m not Shwazzed. You know everything, Shwazzy. Shwa me what you can do.”

“Shut up,” said Zanna.

“Shwat up yourself.” Zanna couldn’t help laughing at the ridiculous riposte.

They gripped a chimney stack and waited for their hearts to slow down. Far off, they could see the rise of tower blocks and the odd shell or vegetable or typewriter-and-fridge roof of UnLondon, but for a long way, it was just foreboding hillocks of slate.

The air was darkening. Deeba leaned into the chimney. Curdle nuzzled her forlornly.

“Oh man, ” Deeba said. She couldn’t stop herself saying, “I want my mum and dad. How do we get down?”

“Why in the name of Unstible,” a loud voice said, “would you want to get down?”

Zanna and Deeba whirled around. Curdle squeaked.

They were surrounded.

There were men and women on the ledges. They wore tough-looking furs and padded boots.

They trotted carelessly on brick ledges, somersaulted like gymnasts, and landed poised on slopes. One man had a baby strapped to him in a harness on his chest. It gurgled happily as he scampered up and down giddying slopes.

“ ‘Get down’ indeed,” the same voice said.

On a roof overlooking them was a tall, athletic, imperious-looking woman. She strode casually, reached a gap between buildings, jumped calmly over it, and landed on her toes. She took hold of an antenna and swung around it.

“You, young grubs, are in the territory of the Slaterunners. So might I ask just what exactly groundlubbers like you are doing in the Roofdom? Because we prefer guests ask before they come in.”

Zanna and Deeba swallowed.

“We’re looking for someone called Badladder,” Zanna said.

“Oh are you?” the woman said, and the Slaterunners laughed. “And what might you want with Badladder?”

“Conductor Jones dropped us here,” Zanna said.

“He had to go,” said Deeba. “He wanted to stay but—”

“We were being chased by grossbottles,” Zanna said. “He said Badladder’d help us. He said he’d owe her one.” The Slaterunners were blinking, surprise breaking through their arrogance.

“What help is it you need?” the woman said.

“People want to stop me,” Zanna said hesitantly. “I don’t know why. It’s because of…this.” She held up the travelcard.

“Shwazzy!” The whisper went through the Slaterunners. “Shwazzy!” “Shwazzy!”

“You’re here?” someone said. “It’s happened!” And: “At last!” “Is Unstible with you?” “Did you bring the Klinneract?”

“I don’t know what any of that means,” Zanna said. “Jones said the Propheseers would explain.”

“We have to get out of here,” Deeba said.

“Will you help me?” said Zanna.

“Of course, ” the woman said. “I can’t believe you’re here. At last. Now the bloody Ess Emm Oh Gee better watch itself!” She vaulted and landed in front of them. “I’m Inessa Badladder. This is Eva Roadshun; Alfred Stayhigh; Jonas Ridgetrotter; Marlene Chimneyvault…”

“I’m Zanna. This is Deeba. Pleased to meet you.”

“The Propheseers live in Pons something,” Deeba said.

“Shwazzy, it’s an honor to be of help,” Badladder said, ignoring Deeba.

“We have to go to the bridge,” said Zanna.

“The Pons Absconditus,” said Badladder. “Of course.”