The two girls crept out into the estate night.

“Quick,” Zanna whispered. “It was over there.”

“This is mad,” hissed Deeba, but she moved as quickly as her friend, in the same half-bent run. “We don’t even have a flashlight.”

“Yeah but we’ve got to look,” Zanna said. “What is going on ?” They shivered a little in the clothes they had quickly put on, looking nervously around them into darkness and halos of lamplight. They headed for the bins, and the hollow full of rubbish where they had seen the impossible spy.

“So it was some sort of remote control thing, innit?” Deeba said as Zanna looked around in the smelly dark. “And maybe…I dunno, maybe it had a camera or something…and…” Deeba stopped, as what she was saying began to sound more and more unlikely.

“Come help me,” Zanna said.

“What you doing?”

“Looking for something,” Zanna said.

“What?”

Zanna poked about in the rubbish, holding her nose as she prodded the overspill from the bins with a stick.

“There’s going to be rats and stuff,” Deeba said. “Leave it.”

“Look,” said Zanna. “See that?” She pointed at one streak among many across the cement of the estate.

The smear, just faintly visible, stretched from the rubbish tip, towards the dark ground-floor windows of Zanna’s house.

“That thing. These are its tracks.”

Zanna got on her hands and knees.

“Yeah, see?” she said. “You can see scratch-marks. Where it’s dug in with its…you know…metal points.”

“If you say,” said Deeba. “Let’s go. ”

“Look. It was watching, or listening or whatever, at mine. Now we can see where it went.”

“We don’t even know what we’re after.” Deeba followed Zanna, who bent carefully over and traced her way through the dark estate. Deeba peered over her friend’s shoulders, trying to make out the tracks Zanna could see.

“You blatantly look like a mad person,” Deeba whispered. “If anyone sees you, what they going to think?”

“Who cares? Anyway, there’s no one. If there was, I’d be out of here.”

“I don’t even see nothing.”

“Marks,” Zanna said. “Tracks.”

She headed into the backs of the estate, between the brown concrete of those huge buildings. They were heading deep into the dead zones behind all the towers, into a maze of walls, bins, garages, and rubbish. Deeba looked around nervously.

“Come on, Zann,” she said. “We dunno where we are.”

“I’ve got a feeling…” Zanna said. She was distracted.

“This way…” she said, glancing down without slowing. In fact, she looked now as if she were following a memory, or an instinct, rather than a trail. She wound between the enormous buildings, lit here and there by inadequate yellow lights.

“I can’t see it,” Deeba said anxiously. “There’s nothing.”

“Yes, there is,” said Zanna dreamily. She pointed, almost without looking. “There, see?” She sounded surprised. “It came this way.” She accelerated.

“Zanna!” said Deeba in alarm, and trotted to keep up with her. “How can you even see that?”

The main road was just out of sight: even at this hour, they could hear traffic. Zanna turned a corner, moving almost as if she were being tugged.

“Wait!” said Deeba, and came up behind her.

In front of them, in the base of one of the monoliths, surrounded by puddles of pretty oily water, below a weakly shining lamp, the girls saw a door. It was ajar. On its threshold, even Deeba could see it was marked with a smear of oil.

“No way,” Deeba said, eyeing Zanna. “You are not going there…”

Zanna stepped inside. Behind her, shouting, “Wait! Wait!” Deeba followed.

“Is anyone there?” Zanna said, not very loud. They were in a narrow corridor below ground level. The only windows were stubby ones by the ceiling, cracked and flecked with cobwebs and fly husks. The one or two bulbs let light out resentfully, as if they were misers who hoarded it.

“We are going, ” Deeba said. “There’s nothing here.”

Pipes and wires ran along the walls, and meters ticked.

“Hello?” Zanna said.

The corridor ended in a huge basement. It must have stretched underneath almost the whole tower block[25] . Along its walls were old tools; there was rope in thick puddles; and sacks; and rusted bicycles; and a dried-out warmed-up fridge. Here and there were faint illuminations, and the light from streetlamps came through the filthy windows. The girls could hear the moan of traffic.

In the middle of the room was a pillar of pipes, where needles jerked up and down on gauges, and pressure was channeled by fat iron taps. In the dead center was an ancient, heavy-looking one the size of a steering wheel. It looked like it would open an airlock in a submarine.

“Let’s go,” whispered Deeba. “This place is scary.”

But, slowly, Zanna shuffled forward. She looked like a sleepwalker.

“Zanna!” Deeba moved back towards the door. “We’re alone in a cellar. And no one knows we’re here. Come on !”

“There’s more oil,” Zanna said. “That thing…that umbrella, was here.”

She touched the big spigot experimentally.

“ ‘…when the wheel turns,’ ” she said.

“What?” said Deeba. “Come on. You coming?” She turned her back. Zanna gripped the wheel, and began to turn it.

It moved slowly at first. She had to strain. It squeaked against rust.

As it went, something happened to the light.

Deeba froze. Zanna hesitated, then turned the wheel a few more degrees.

The light began to change. It was flickering. All the sound in the room was ebbing. Deeba turned back.

“What’s happening?” she whispered.

Zanna tugged, and with each motion the light and noise faltered a moment, and the wheel turned a little farther.

“No,” said Deeba. “Stop. Please.”

Zanna turned the valve another few inches, and the sound and light shifted. All the bulbs in the room flared, and so, impossibly, did the sound of the cars outside.

The iron wheel began to spin, slowly at first, then faster and faster. The room grew darker.

“You’re turning off the electricity,” Deeba said, but then she was silent, as she and Zanna looked up and realized that the lamplight shining through the windows from outside was also dimming.

As the light lessened, so did the sound.

Deeba and Zanna stared at each other in wonder.

Zanna spun the handle as if it were oiled. The noise of cars and vans and motorbikes outside grew tinny, like a recording, or as if it came from a television in the next room. The sound of the vehicles faded with the glow of the main road.

Zanna was turning off the traffic. The spigot turned off all the cars, and turned off the lamps.

It was turning off London.