"Right," said the young man, "this looks like it." On his shoulder sat a large rat, eyes full of panic. It was bred for night work in tight spaces and the walkway and wide moat gave the animal agoraphobia. So the rat didn't really care which turret its owner chose so long as he took them somewhere darker, preferably with a roof and walls on all sides.

The young man assumed his pet ran some kind of simpatico system but the truth was stranger: an ancestor of the rat had been coded for basic language skills. To say that Null understood more than it said was ludicrous because -- obviously enough -- the rat said nothing; but Null could comprehend a vocabulary of about fifty words and construe probable meaning from the tone of many others.

"Up here, I guess..."

Long lengths of bamboo scaffolding fat as a child's leg had been erected against the north-east turret and lashed together at the cross points with rope. When the young man got closer, he realized that the uprights actually grew from the dirt while the crossbars were held in place by vines which had grown up the side of the scaffolding. A barge loaded with roof tiles had been tied to a wooden pontoon.

Climbing the rig was simplicity itself, so while the rat shut its eyes the young man made his way to the top, walked a plank between scaffold and crenellations and dropped into a different walkway, one that ran from corner turret to corner turret around the walls of the Purple City.

Below him were the eastern pavilions, storehouses and the Qianlong Gardens. Walled areas within other walled areas within the walls of the palace. A vast and elderly eunuch waddled from beneath an arch and stopped to watch a gardener's child roll a hoop from one side of a tiny courtyard to the other. Behind the chamberlain came two younger eunuchs, probably not much older than the man watching from the top of the wall, although both had the soft, child-like faces of those who'd been castrated at least a year prior to reaching puberty.

And though none of these three looked up to where the servitor stood on the upper walkway, a billion or more watchers saw him reach out to soothe his rat, explaining his plan in simple words until the rat began chattering to itself.

As far as the Library could tell, the Emperor was not aware of this rise in interest from those watching, which in itself was worrying, not because sulking was unknown to emperors but because the Librarian expected a stronger link between the watchers and one raised to the Celestial Throne.

Yet, with Zaq, this link was no stronger than the link between those watching and the young man now walking calmly into the north-east turret, smiling to a guard and starting down the tower's great wooden stairs.

Since it was impossible for a servitor to manifest the same level of empathy as the Chuang Tzu, the Librarian dropped this anomaly down a level, allowing a subroutine to extrapolate all possible reasons simultaneously and arrive at no single explanation logical enough to pass back.

"See?" said the young man. "Nothing to it."

The rat wisely stayed silent.

Man and rat might as well have been invisible to the inhabitants of the Forbidden City, for all the attention they attracted as they left the turret and crunched along the wide stretch of gravel between the boundary wall of the eastern pavilions and the northern wall of the Forbidden City itself. Two minor eunuchs even stood aside to let the servitor pass.

"Thank you."

"You're welcome." It was clear from the ennui in the taller eunuch's voice that he'd barely registered the existence of the man for whom he just stepped aside.

"Whatever..." Tucking the rat into the sleeve of his coat, the man cut through the north gate of the Imperial Garden, exited through the southern gate and passed under an arch into the Emperor's inner court, at the centre of which stood a marble dais and the three private pavilions.

As with all areas within the Forbidden City, the inner pavilions were circled by their own walls. Only these walls were formed by a continuous line of offices, bedrooms for concubines, a kitchen and endless store-houses for gifts from the various ambassadors, mostly unopened and some going back ten or fifteen centuries.

A chef was waddling towards him so the man stepped hastily back, out of the chef's line of sight. Then he counted to a hundred, which he managed by counting slowly to ten and then counting to ten again and again, starting with the little finger of his left hand and finishing with its mirror image on his right.

"Where's the Master Chef?"

The servitor fired off his question the moment he stepped out of the steam, materialising beside a bubbling cauldron of crab broth, into which a tall sous chef with a hollow face dropped intricately wrapped dim sum.

"It's just," continued the servitor, "that His Celestial Excellency requires something to eat..."

Chang San, whose unfortunate nickname was Old Rat, blinked and disdain gave way to shock, followed quickly by envy and finally careful consideration.

"I'll arrange something," he said, as over his shoulder another half a billion watchers understood instantly that this was exactly the chance for which the sous chef had been waiting. "You can go," he told the younger man, "leave this to me."

"I'm afraid not." The young man shook his head, appearing almost contrite. "I'm to take it to His Celestial Excellency myself." He glanced into the copper pot boiling on a range beside Chang San. "Shrimp?"

"Pork," said the sous chef.

"They look perfect," said the servitor. "Guaranteed to touch any emperor's heart."

Shrewd eyes watched the younger man. "You will tell His Celestial Excellency that Chang San prepared the dim sum, won't you?"

"Of course," said the servitor. "You have my word." He looked beyond the boiling cauldron to busier cooking ranges. Chilli and ginger sharpened the air, while dancing flames flash-flared like furious ghosts above red-hot woks and oily smoke caught in his throat.

All possible meals were being prepared at all possible times. Unfortunately it was weeks since the Emperor had eaten any of them.

"I'll need a tray," the servitor said.

For a second it looked as if Chang San might simply yell across the kitchen to one of the boys, but though the chef opened his mouth to shout he thought better of the idea. Nodding to himself, Chang San told the servitor to stay where he was.

When he returned it was with a tray edged in red-lacquered ebony and inset across the base with a thin, almost completely translucent slab of mutton-fat jade.

"Treat this carefully," said Chang San, handing his prize possession to the waiting servitor. "It belonged to the previous Chuang Tzu."

Major Abbas waited for a cart to get out of his way and then stepped over the legs of a beggar as if she didn't exist. The boy beside him stopped for a second, but only because the Berber woman was feeding her child, pendulous breast sucked hollow by the infant's appetite.

On the western edge of Djemaa el Fna, where orange-juice stalls lined the sticky blacktop, Moz risked a glance at a fruit seller and then flicked his eyes sideways, his gaze drawn by a passing handcart piled high with dates, dragut noir from the look of them.

"Hungry?" Major Abbas asked.

"Thirsty," Moz said.

The police officer pushed his way through the crowds to the nearest stall. "One juice," he said, then indicated a French tumbler blown from greenish glass, bubbles suspended in the sides. "Make it large."

And together Moz and the police officer waited in the gathering dusk as the stallholder pulped half a dozen oranges and strained the juice through a plastic sieve. Smoke from a recently erected rotisserie stall competed with red grit to fill the air. As ever, tables and chairs were being set out in the busy centre of the square, arranged around kitchen carts, their hand-scrawled menus taped to metal posts that kept overhead canopies in place.