'There,' I told him. I could now hear the familiar gurgling roar of water, the River Cam funnelling between the pylons of the bridge. 'See it? The Bookbinder's Arms.'

But Crump made no reply. Jaw tightly set, he glanced over one of his enormous shoulders again, shook the reins, and the horses moved forward at a swift trot. Perhaps he hadn't heard me over the roar of the water. I pointed at the building and then made to tap him on the forearm-we were nearing the end of the bridge and would pass the inn at this pace-but my fingers touched something cold and hard instead. Looking down, I saw the pistol gripped in his right hand.

'Giddap! Go on! Giddap!'

The horses plunged forward across the bridge so quickly that I was almost thrown from my seat. When I righted myself I heard Crump's oath and, turning my head, saw that we were no longer alone. The mud-spattered coach-and-four was approaching from the opposite side, blocking our path, and ahead of it a blue roan with a horseman astride was charging towards us.

I turned in confusion to Crump. He grimaced, cursed again, then raised his pistol in the air and pointed it at the figure rearing in the stirrups. The roan veered sideways into the stone balusters as the weapon discharged itself with a bright shower of sparks, stinging my left cheek. Our own horses bolted forward, panicked by the report, the coach swaying wildly behind them. I clung to the edge of the seat as Crump fumbled with the reins and another cartridge for his pistol. In a few more seconds we would draw level with the other coach.

'For God's sake help me!' Crump was thrusting the pistol and its cartridge towards me. The hub of one of our wheels ground against the balustrade, and our heads came together as the coach lurched violently sideways. 'They'll kill us!'

But I didn't take the pistol, which clattered on to the bridge behind us. Instead I recoiled from him as the coach righted itself, then I twisted round in the seat and hoisted myself with a clumsy bound on to the rocking coach-top, where I crouched for a second on my haunches, gripping the edge. Then, without heeding Crump's shouts or looking downwards, I leapt over the balustrade and into the swirling din of the rain-swollen Cam. But as I hit the waters with a splash and was sucked below the surface, then through the middle arch, then downriver past the Bookbinder's Arms, it wasn't the thunder of the flooded river I was hearing but the echo of Crump's wooden teeth clicking together like rattlebones.

For I had remembered, at long last, where I'd seen him before. But then for a long time, as the current carried me downstream, I remembered nothing at all, because suddenly the whole world had gone black and silent.

From Magdalene Bridge the River Cam flows northeast towards the Isle of Ely, several miles below which, on the edge of the peat fens, crosscut by ancient Roman drainage canals, its waters run into the Great Ouse and then seaward to the Wash, thirty miles to the north, where they flow towards a desolate horizon. With the day's downpour the fens were even more flooded than usual, and that evening the river's current was turbulent and swift. How many miles it might have swept me downstream I had no idea. I only know that I awoke sodden and chilled on the floor of a lighter that was being poled against the current by a fenman on his way to market, an ancient turf-cutter named Noah Bright. Stars were reeling overhead and muddy embankments wavering past. I coughed up a lungful of water and fetched my breath in ragged gasps. It might have been hours or even days later.

Of the journey back to Cambridge I have only the vaguest memories: the old fenman leaning on his pole; the motion of the lighter in the water as a dark riverscape slid over the gunwale of the boat; the sweet odour of the sun-dried peat against which my cheek was pressed. Bright kept up a spirited monologue as he poled us along, though what he might have been talking about I have no idea, for I barely listened or responded. I was thinking all the while about Nat Crump, about what I had seen when our heads clashed together on the bridge: the set of wooden teeth bared like a cur's with fear and anger.

An accident in Fleet Street. Cart-horse dropped down dead, sir…

The discovery had been a shock. Even now I had no idea what to make of it. But Crump had been the driver of the hackney-coach in Alsatia, that much I knew at once. Crump was the one who took me on that apparently fortuitous detour to the Golden Horn. I was as certain of that, at this moment, as I was of anything.

An accident in Fleet Street…

For I could no longer know anything, I realised, except that a few days ago someone named Nat Crump had followed me to Westminster, to the Postman's Horn, where he picked me up from the street, to all appearances at random, and then delivered me to within sight of the Golden Horn, also apparently at random. But the journey must have been carefully planned and executed so that the elaborate design would appear as an accident, a coincidence, a rare piece of good luck. Which meant that everything that had happened since the first trip to Alsatia, as well as everything that had followed so smoothly from it-the auction, the copy of Agrippa, the catalogue-had also been staged. As, of course, had the journey to Wembish Park. I was being led astray, coaxed into ever-deeper and more dangerous waters. Even if the house actually existed I had no doubt that it, like all else, would be nothing more than a blind. But a blind for what? For whom?

We seem to have reached a dead end…

And the loquacious turf-cutter, Noah Bright, who was rearing above me in the stern? What of him? He seemed to be watching me closely as he spoke, bending upon me a pair of eyes as bright and alert as an old pointer dog's. I had managed to explain that I was a bookseller from London, Silas Cobb by name, who had come to browse among the shops and stalls of Cambridge's Market Hall, but who had toppled into the river after enjoying the hospitality of one of the town's numerous taverns. I had no idea if he believed my hasty fibs-or if I could trust him. Suddenly I was suspicious of everyone. I wondered if the old fenman wasn't yet another Crump or Pickvance, an actor brought on stage to play a part, a marionette whose strings were twitched from behind canvas flats by someone else. Had he found me in the river only at random, by pure chance? Or was even my leap overboard under some sort of precise control, determined by a set of indices whose author and purpose remained a mystery? I wondered where the limits of this control might lie. I wondered if Biddulph with his tales about the Navy Office and the Philip Sidney had been arranged for me like everything else. If the graffiti had been scrawled on the walls of London and the curiosities placed in their dusty cabinet for my eyes alone…

'What the devil…?'

The lighter had skidded sideways in the current, yawing frantically to starboard. Water splashed over the gunwales and the load of peat wobbled unsteadily beside me. I raised my head to see that Bright had ceased poling and was squatted in the stern, peering anxiously across the flooded river. Turning my head I saw the faint lights of Cambridge in the waters ahead of us. We must have been a good mile or more north of Magdalene Bridge. The lantern teetered on the thwart and threatened to tumble into the waves. I turned my attention back to Bright, a wave of gooseflesh creeping across my nape and shoulders.

'What is it?'

'Over there,' he whispered, nodding to the embankment. 'There's something on the riverbank.'

I turned my head again and saw a dark shape half-hidden among the waterlogged sedge: what looked like some sort of amphibious creature that had crawled halfway out of the water. Light from the lantern played towards it as Bright sank the pole in the mud and pushed off, carefully drifting the nose of the lighter across the treacherous current. He almost lost his balance but managed to hold the course, ruddering with his pole as we wallowed in the onrushing water. A few seconds later the keel slid with a soft rasp into the mud. I could see an arm outstretched in the sedge. Bright raised his pole from the water with a grunt.