“The secret,” said Inessa Badladder, “is not to look down.”

“I wasn’t going to,” said Zanna.

The Slaterunners led Zanna and Deeba laboriously over the roofs. They threw up rope bridges over the gaps of streets and guided the girls over, whispering, “Look straight ahead.” Once there was that sudden plaintive bleating again. Zanna and Deeba froze.

“Don’t worry,” Inessa said. “It’s just a trip.”

“A what?”

Clattering elegantly down the slates came a line of goats, staring with their strange eyes.

“That’s what you call a group of them,” Inessa said. “A trip of mountain goats.” The animals watched them go. Deeba stared back, thinking she had seen something flit pale and fast behind the goats, but only the chewing herd moved.

“I can’t understand how you lot can live down there, without this freedom,” Inessa said. “Walled in. I’m third-generation groundless. My mother never touched down, nor my grandmother. My great -grandmother once had to. It was an emergency. The roof was on fire.”

“Look,” said Zanna, and the two girls paused in their exhausting climb. As it set behind the bizarre silhouettes of UnLondon, the UnSun was rainbow-shaped, an arch of light.

Flocks of birds gathered, circled, and separated into species. Swirls of pigeons and starlings and jackdaws headed towards the tall, thin rectangular towers that dotted the abcity. The buildings’ fronts broke with thousands of drawers, into each of which one bird flew. The little compartments slid shut. “They are chests of drawers!” Deeba said. “That’s where the birds sleep!”

“Of course,” said Inessa. “You couldn’t just have them all over the place; it would be chaos.”

The UnLondon moon rose, and Zanna and Deeba stared at it in astonishment. It was not a circle, nor a crescent. Instead, it was a perfectly symmetrical spindle, pointed at the top and the bottom, like the slit in a cat’s-eye.

“Our way will be lit,” Inessa said, “by the light of the loon.”

Stars appeared in the dark. They were not still like the stars of London: they crept like luminous insects every which way. There was a sputter as streetlamps came on in the streets below and orange light shone up from the gaps between the roofs.

“What was that?” said Deeba. She pointed past the edge of a gutter, into one of those narrow unseen alleys.

There was nothing there. “I swear I’m going mad,” Deeba muttered. “I keep thinking I see something.”

The girls followed their guides, clambering onto an apex, and into a sudden glow. The light source came into view. It was only a few streets away, just beyond the edges of Roofdom.

“It’s…” Deeba whispered.

“…beautiful,” Zanna said.

For a moment it looked like a fireworks display, the most amazing, huge, impressive one ever. But it wasn’t moving. It was an enormous tree of firework-bursts, stuck together and motionless.

The trails of several rockets made a trunk. They jutted off at various heights in boughs of light and curved down like a willow tree. Colors filled the rocket-trail branches like leaves, in glimmering red, blue, green, silver, white, and gold. Catherine wheels and the bursts of Roman candles, the buds of sparklers hung motionless and silent like fruit.

“The November Tree,” Inessa said.

“This is a good time to see it,” Inessa said. “It was a bit forlorn a couple of weeks ago. Almost at the end of its life. But Guy Fawkes Night is springtime for the November Tree.”

Fireworks were obsolete the instant after they ignited. Every November on Fireworks Night, the most choice effects of the most impressive displays in London would seep through into UnLondon as they became moil, and blossom into the November Tree. Over the year the tree would dim, shedding its glow and colors, until by November the fourth it was little more than a skeleton of smoke trails.

Then the cycle would begin again. The rejuvenated tree would light up the night.

Several small, crackling shapes scampered up the November Tree. Squirrels. Their claws gripped the solid glow. Their coats smoldered, but they did not seem uncomfortable.

“This is where the toughest red squirrels moved,” Inessa said. “After the grays came. They’re fireproof, though they keep that to themselves. Once or twice a gray makes it here and tries to follow them. Don’t get very far.” She mimed an explosion.

“I wish I had my phone,” Deeba whispered to Zanna. “I want to take a picture.”

At the shining highest branches, something swooped. Most of the birds were gone from the sky now, but above the tree was one that had not joined any of the throngs. It circled.

“There’s something weird with that bird’s head,” said Deeba.

Its skull bulged wrongly. The November Tree’s light glinted from its eyes.

“You’re right,” said Zanna. But it wheeled off too fast to see— into a last, sleepy flock of ducks— and disappeared.

“What was that?” said Zanna, but she was interrupted by Inessa’s shout.

“Hey!” Deeba and Zanna turned and screamed.

Creeping without sound from around a chimney pot behind them, hunched over like a monkey, draped in what looked like a curtain, was Hemi. He was only inches away. He was reaching out, his fingers actually touching Zanna’s pocket.

He leapt up as the Slaterunners launched themselves at him, his look of concentration becoming one of alarm. Hemi scrambled up and down the roofs to get away, Inessa’s tribe quickly after him. They gained on him, but he reached the edge of a roof, gathered himself, and jumped, the cloth he wore flapping like a cape, down into the dim gap between the buildings and out of sight.

When his pursuers reached the building’s edge they looked into the alley in both directions, and shook their heads.

“He’s gone,” one shouted.

“Who was that?” Inessa said. Deeba and Zanna were shaking.

“A ghost,” Deeba managed to say.

“That was him off the bus,” said Zanna. “He’s following us.”