Sleep restored some of his optimism. And this was fortunate, because he now had to head into a grimy London suburb and spend the morning thinking like Robin Crake.

He loved boarding the trains at Oxford Station. He had been free to leave the city for seven months now, but it never lost its novelty. He liked to see those spires receding into the distance as the train pulled away, until they were just black thorns on a plant he would never have to trouble himself picking.

Of course, the pleasure was mixed with a few misgivings this morning. Leaving the town at a time like this - leaving Ellini unguarded when she was hell-bent on challenging Myrrha, and when Elliott Blake was hanging around, doing the obscenely brilliant things he did with a piano, felt wrong. But what else could he do? How else could he help? He'd go mad if he didn't try to do something.

Twenty years had changed Camden Town beyond recognition, apart from the soot. Jack looked for the cottage by the lock, only to find that it had been torn down to make room for a warehouse, plastered with advertising bills at different heights, so that the marvels of Doctor Thomas's World-Famous Boot Blacking would be apparent to everyone, whether they were short or tall.

The Old Mother Redcap was now called The World's End, which seemed unreasonable, because it was clear when you stood on Camden High Street that the world stretched on for miles in every direction, and would never tire of trying to sell you things.

There was nothing very extraordinary opposite the pub - just a shonky shop and a cabinet-maker's. There was also a coffee-vendor's stall, topped with two large, five-gallon cans containing the tea and coffee. The coffee-vendor was chatting to a woman in an apron, who was holding a baby in a business-like way, as if it was a basket of laundry.

The spring was still in Jack's step, in spite of everything. He felt healed - sharp - stretchy. And this was so like the neighbourhood he'd grown up in. He didn't have very good memories of the neighbourhood he'd grown up in, but it was still a world he knew. He could disappear here. He could wear the streets like an oilskin coat.

There were lots of men in cloth caps queuing outside the stall, probably on their way to work, waiting for their morning brew, or a thick wodge of bread and butter to serve as breakfast.

Jack joined the queue and bought a bacon sandwich, wrapped in greasy wax-paper, which he chewed thoughtfully as he wandered up the High Street, trying to picture what had happened that day, twenty years ago.

He knew Sita had been the first murder, although he couldn't remember when he'd picked this up. It made sense, both from what he knew of Robin, and of Ellini. Of course she would talk proudly of her clever little sister, and so of course Robin would have seen her as his biggest rival for Ellini's affections.

And, for the first murder, Robin would have wanted her to be unsure whether it had really been murder at all. That would prolong Ellini's agony and ensure he got to spend more time with her. He would have been walking with Sita, perhaps, and pretended to stumble.

The uncertainty of it must have bothered him, though. Even if he found a deep enough shaft, even if he timed it just right, there was always the possibility that she might survive. And what would he do then? Creep into the sick-room and smother her with a pillow? Hardly the dramatic spectacle he'd had planned.

Jack's stomach was tight, but his mind was racing. He hated, hated, hated that he could do this - that he could put himself so easily into Robin's shoes. But, at the same time, it was so beguiling - to think without consequences, to put your needs at the centre of the universe, to do what you did so well without a second thought. How free he must have felt when he was planning those murders! How dire and despicable, how creative...

Jack got to the end of the High Street and turned back, causing a man in a lounge suit to walk into him. And that was when he heard it. It could have been anything - the sound of running water was hardly out of place in a city. It could have been a flushing privy, or a burst sewer-main, or a fountain. He wouldn't have thought anything of it if it hadn't been for Elsie's description of Sita, falling through endless night, with the roar of a river or a waterfall nearby.

He looked around, and failed to see anything that could account for it - no fountains, no open windows that might have housed fancy water-closets, no workmen clustering round a burst pipe in the road. But it was louder now - now that he was listening - and seemed to be coming from the street to his left.

He followed it into an alley, lost it outside a penny-gaff, retraced his steps, turned into another alley, and almost tripped over the source of the noise: a small sewer grating outside a saloon.

The light - such as there was of it in this warren of alleys - picked out the glint of water maybe six feet below the grate. But there was too much of it for a sewer-shaft - even a sewer-shaft that had been swollen by a week of non-stop rain. It was a river. It rushed and gurgled and occasionally splashed up onto the pavement, swelling the fetid puddles at his feet.

He knew something about this, didn't he? London's lost rivers, forced underground - built over and incorporated into the sewer system. This one would be the Fleet, yes? Coming down from Hampstead Heath and wending its merry way towards the Thames?

Thoughtfully, he walked back to Camden High Street, and paced around until he found what he was looking for: another sewer-grating, on the road opposite The Mother Redcap. You couldn't hear the rushing of water from this one. The drop must have been deep. Deep enough to break a young girl's neck, if you had one you wanted to get rid of.

So Sita fell down there and into the Fleet, and it carried her - where? Straight into the demon realms? If everything that fell into the London sewers was carried into the demon realms, wouldn't hell be crammed full of rubbish, dead dogs, dish-rags, and excrement? Perhaps it was. Perhaps that was why it was hell.

But Elsie had described an excavation - extensive digging - not a single sewer shaft. And how could Robin have got away with it, in broad daylight, on a crowded street?

But twenty years ago... oh yes. Twenty years ago, they would have been building the new interceptor sewers, the ones that incorporated the Fleet into their tunnels. There would have been workmen and bricks and piles of refuse everywhere.

A young girl walking through that chaos would have had to take the hand of the nearest adult. Perhaps that adult had even shouted for help and organized search-parties. Perhaps the workmen grabbed his shoulder to prevent him from going after her, saying there was no point, it was a sheer drop, fifty feet or more into the Fleet, no-one could have survived it, no point your dying too, sir.

What a bastard. What a bastard! How carefully he had planned it - how romantic it all seemed. Had Ellini sobbed onto his shoulder? But had there been some doubt - the doubt Robin had intended to plant there - worming into her like a tiny screw, that he could tighten or loosen as he pleased?

The more Jack thought about it, the more he felt that the way Robin had tormented her was beyond even Robin's depraved imagination. He didn't have the wit to stage something so story-book perfect - something that could have come straight out of a novel by Mrs Gaskell or George Eliot. And when would he have had time to read the Ramayana, and find out how Sita died? It was Myrrha. It was all Myrrha. She might as well have scratched her signature into the grating at Jack's feet.

And then, worst of all, she had left Ellini alone. Left her to heal, left her to find love - as if she wanted her to be whole before she ripped her to pieces again.

Jack almost sagged with the frustration of it. Ellini was in danger and he couldn't help her. He didn't know how. Would finding her sister change anything? Even if it would, how could it be a priority? How could he let it take him away from Oxford, where he could watch her, and-

What? What was the point of watching her? What could you do, even if you saw Myrrha coming? You don't know how to fight her. You don't even know what she is.

And then he came up with a plan. The barest sliver of a plan - not to win, but to put off the fight until he was less certain to lose. To buy himself more time. And Ellini would like this plan, because it involved danger for herself, and the opportunity to help someone who didn't deserve it.

It was a liberating thought - not comforting, but liberating. It allowed him to take a step back and think about the problem that had brought him here.

He had still not found Sita. It was no good going back to Elsie and telling her she had fallen into the River Fleet, and must have got into the demon realms from there. That didn't narrow anything down. That was wide as a bloody Parisian Boulevard, not a poky alleyway in Camden Town.

The only way to get more information was to go down there, and walk - or rather, wade - the dark and fetid miles of the London sewer system. In the February cold. With the turds and the dead dogs and the rats.

He delayed as much as he could, calling it prudence rather than prevarication. It would be pitch black down there as soon as he was out of sight of the grating. He would need something to light the way - perhaps even ropes and pick-axes if he got into difficulty.

He bought a lantern and a packet of waterproof matches in one of the poky shops off Camden High Street. A compass would have been useful too, but nowhere seemed to stock them. This was a city, they said, not a bloody jungle. If you wanted to know which way was East, you just looked for the nearest east-bound train on the Metropolitan Railway.

Then, sullenly, he made his way back to the alley. It would be easier to get in there, without attracting attention. And he liked the idea of running water - it seemed cleaner, somehow.

Poor Sita, he thought, as he looked through the grating to the dark, rushing water below. What a place to spend twenty years slowly dying, even if the twenty years only seemed like twenty minutes. He hoped she was unconscious. He wished he could be.