"We're going to have to walk home."

"Fine," Cal replied unquestioningly, his head back as he stared at the stars, losing himself in the canopy of the sky. "At last I've seen them," he whispered to himself.

A helicopter drifted across the horizon.

"Why's that one moving?" he asked.

Will felt too tired to explain. "They do that," he said flatly.

They set off, keeping close to the bank so as not to be noticed, and almost immediately came upon a set of steps leading up to the walkway above. It was next to a bridge. Will knew then where they were — it was Blackfriars Bridge.

A gate blocked the top of the steps, so they hastily clambered over the broad wall beside it to reach the walkway. Dripping water on the pavement and freezing in the night air, they looked around them. Will was seized by the dreadful thought that even here the Styx might have spies watching out for them. After seeing one of the Clarke brothers in the Colony, he felt that he couldn't trust anybody, and he regarded the few people in the immediate area with mounting suspicion. But nobody was close., with the exception of a young couple walking hand in hand. They strolled past, so involved with each other that they didn't seem to pay the boys or their huge cat the least bit of attention.

With Will taking the lead, they climbed the steps to the bridge itself. Arriving at the top, Will saw that the IMAX cinema was to their right. He immediately knew they didn't want to be on that side of the river. To him, London was a mosaic of place, each familiar to him from the museum visits with his father or school expeditions. The rest, the interconnecting areas, were a complete mystery to him. There was only one thing to do: trust in his sense of direction and try to head north.

As they turned left and quickly traversed the bridge, Will spotted a sign to King's Cross and knew instantly that they were heading the right way. Traffic passed them as they arrived at the end of the bridge, and Will paused to look at Cal and the cat under the glow of a streetlight. Talk about three suspicious-looking lost souls — they stuck out a mile. Although it was dark, Will was painfully aware that a pair of young boys soaked to the skin and wandering the streets of London at this late hour, with or without a giant cat, were likely to attract attention, and the last thing he needed now was to be picked up by the police. He made an attempt at concocting a story, rehearsing it in his mind, just in case it happened.

'ello, 'ello, 'ello, the pair of fictitious policemen said. What 'ave we 'ere, then?

Uh… just out walking the… the… Will's imagined response came to a faltering stop. No, that wouldn't do, he had to be better prepared than that. He started again: Good evening, officers. We're just taking the neighbor's pet for a walk.

The first policeman leaned in to peer curiously at Bartleby, his eyes narrowing as he grimaced in open distaste. Looks dangerous to me, son. Shouldn't it be on a leash?

What is it, exactly? the second imaginary policeman chimed in.

It's a…, Will began. What could he say? Ah yes… It's very rare… a very rare hybrid, a cross between a dog and a cat called a… a Dat, Will informed them helpfully.

Or is it a Cog, perhaps? the second policeman suggested drily, the glint in his eye telling Will he wasn't buying a word of it.

Whatever it is, it's bloody ugly, his partner said.

Shhh! You'll hurt his feelings. Suddenly, Will realized he was wasting his time with all of this. The reality was that the policemen would simply ask for their names and addresses, then radio in to double-check them. And they'd probably be found out even if they tried to give false ones. So that would be it. They'd be taken back to the station and held there. Will suspected he was probably wanted for abducting Chester, or something equally ridiculous, and would likely as not end up in a juvenile detention center. As for Cal, he would be a real conundrum — of course, there wouldn't be a record of him anywhere, no Topsoil identity whatsoever. No, they'd have to avoid the police at all costs.

Perversely, as he contemplated the future, there was a part of him that almost wanted them to be stopped. It would remove the dreadful burden that at the moment lay squarely on his shoulders; he glanced at the cowed figure of his brother. Cal was a stranger, a freak in this cold and inhospitable place, and Will had no idea how he was going to protect him.

But Will knew if he turned himself in to the authorities and tried to get them to investigate the Colony — that's if they believed a runaway teenager in the first place — he could be risking countless lives, his family's lives. Who knew how it would end? He shuddered at the thought of the Discovery, as Grandma Macaulay had called it, and tried to imagine her being led out into the daylight after her long subterranean life. He couldn't do that to her — couldn't even bear the thought of it. It was too big a decision for him to take alone, and he felt so terribly alone and isolated.

He pulled his damp jacket around himself and hustled Cal and Bartleby down into the underpass at the end of the bridge.

"It reeks of pee down here," his brother commeted. "Do all Topsoilers mark their territories?" He turned to Will inquiringly.

"Uh, no… not usually. But this is London."

As they emerged from the underpass and back onto the pavement, Cal seemed confused by the traffic, looking this way and that. Coming to the main road, they stopped at the curb. Will gripped his brother's sleeve with one hand and the cat's hairless scruff with the other. Crossing when there was a lull, they made it to the traffic island. He could see people peering curiously at them from passing cars, and a white van slowed down almost to a halt right beside them, the driver talking excitedly into his cell phone. To Will's relief, it sped off again. They crossed the remaining two lanes and, after a short distance, Will steered them into a dimly lit side street. His brother stood with one hand on the brick wall beside him — he looked completely disoriented, like a blind man in unfamiliar surroundings.

"Foul air!" he said vehemently.

"It's only car fumes," Will replied as he untied the thick string from his light orb and fashioned a slipknot leash for the cat, who didn't seem to mind one bit.

"It smells wrong. It must be against the laws," Cal said with complete conviction.

"Fraid not," Will answered as he led them down the street. He would have to stay off the main roads and keep to the backstreets as far as possible, even though it would make their journey even more difficult and circuitous.

And so the long march north began. On their way out of central London they only saw a single police car, but Will was able to usher them around a corner in the nick of time.

"Are they like Styx?" Cal asked.

"Not quite," Will replied.

With the cat on one side and Cal twitching nervously on the other, they trudged along. From time to time his brother would stop dead in his tracks, as if invisible doors were being slammed in his face.

"What is it?" Will asked on one of these occasions when his brother refused to move.

"It's like… anger… and fear," Cal said in a strained voice as he glanced nervously up at the windows over a storefront. "It's so strong. I don't like it."

"I can't see anything," Will said as he failed to make out what was troubling his brother. They were just ordinary windows, a sliver of light showing between the curtains in one of them. "It's nothing, you're imagining it."

"No, I'm not. I can smell it," Cal said emphatically, "and it's getting stronger. I want to go."

After several miles of tortuous ducking and diving, they came to the brow of a hill, at the bottom of which was a busy main road with six lanes of speeding traffic.