Her last words were lost to me as the wall beside us buckled and more plaster toppled from the ceiling, striking me a glancing blow on the shoulder. I reeled sideways and fell flat for a second time. When I picked myself up, sodden and gasping, I groped for Alethea's hand; but by then she had already disappeared down the corridor. Somewhere at the end of it, in the laboratory, the dozens of glass vials were ringing their alarum.

And now I know what I must do…

Fear gives us wings, they say. But it is also, as Xenophon claims, stronger than love. I must confess that my thoughts were no longer for the books, or even Alethea, but only for myself as I rushed along the corridor a few seconds later. My frantic claudications echoed against the sodden plasterwork until, skidding wildly, I reached not the laboratory but the top of the staircase, which I realised had been my true destination. I hesitated at the sight of it, surprised to have negotiated my way so easily through the maze of corridors. But the marble steps were treacherously slick, and as I began the descent my dizziness returned. From the top step I could see almost the whole of the atrium, the whole dreadful tableau of death and ruin spread before me. The oval looking-glass in the atrium had been knocked over, its cracked face now reflected the gap in the ceiling where the chandelier had broken free. The chandelier itself lay nearby, in the middle of the floor, a mangled bronze bird. Beyond its wreckage I could see Phineas lying on his belly beside the door, his arms flung wide.

There were no more sounds from the laboratory-no ringing vials and no cries for help. For a moment I wondered if I should return for Alethea, but then, gripping the banister, I continued my cautious descent. I was not prepared to die, I told myself, for the sins of Sir Ambrose Plessington. Through the open door I could see that the rain had finally stopped. The wind had steadied and the sun was threatening to appear. Such is the mockery of fate. As I crossed the atrium, my boots crunched the shattered crystal. I felt palsied and unstable until I realised that the floor was trembling underfoot. The blood had spread outwards from Phineas's prostrate body like the tendrils of a bright, submarine plant. I had just stepped past the gaudy slick when I heard a shout and then saw a lone figure in the library doorway, dressed in black. I caught a last view of the felled shelves and the chaos of the sodden masses on the floor before rushing through the doorway and into the dun-coloured light.

The horses, spooked by the commotions, tossed their heads in alarm and shied backwards as I flung myself towards them. The park, half-waterlogged, wavered before me, reflecting a lurid sky. I thought of boarding the coach and so making my escape, but there was no time. I could hear my pursuer shouting in Spanish, while another figure had appeared from round the side of the house, near the physic garden. So I began to run instead, fleeing in the opposite direction, towards the hedge-maze. Perhaps I had visions of drawing the killers away from Alethea-of fulfilling for one last time the task for which I was hired. Had it not been my rash flight from London that brought them to Pontifex Hall in the first place? It was a foolish, fantastic notion: I with my crippled foot and wheezing lungs was no match for either of my pursuers, the second of whom I saw was Sir Richard Overstreet. But as I approached the maze I risked a second glance over my shoulder and saw a deep furrow open in the ground behind me, a long trench running across the park, from the cress-pond towards the coach-and-four.

In retrospect the crevice seems a cataclysm of near-biblical dimensions, perhaps even a miracle, if miracles can be so reckless and tragic. The rear wheels of the coach were swallowed first. The ground trembling beneath them crumbled and the coach tipped backwards before heeling in the trough, which had widened to more than six feet and filled with water as the subterranean current burst to the surface. The horses' hindquarters shimmied for a second and then disappeared. The first of my pursuers, the man in black, stopped short at the brink and stumbled. He stared across at me, aghast and amazed, as the reddish earth collapsed and the chasm yawned ever wider. Then he, too, disappeared into the gaping jaws.

I whirled and kept running. The air was tart with privet and hedge-mustard, whose overgrown branches clawed at my cheeks and shoulders as I dived into the maze and swung left into an even thicker gauntlet of wet branches and sharp holly leaves. Puddles splashed underfoot. Through a small gap in the hedge I glimpsed Sir Richard, his pistol in hand as he dashed towards the entrance to the maze. Another fork. I turned right, then left, threading my way inside the sinuous passages. At one point I tripped on a root and, raising myself, discovered a pair of hedge-clippers abandoned in the undergrowth. I picked them up-the blades were rusty but still sharp-and again took to my heels.

It must have been another minute or two before I heard the scream. By this point I had reached the centre of the labyrinth, a small, scrubby patch of ground on which had been placed a wooden bench, rotted by the elements. I could hear Sir Richard crashing along the paths and realised he must be following my footprints through the mud. Yet another trail that had betrayed me. He would soon catch me-if the hedge-maze wasn't swallowed first, for the ground was trembling and shaking like a stonemason's yard. When the shriek broke the air I was gripping the handle of the clippers and backing into the pruned branches, preparing for a passage of arms. Looking up I glimpsed, above the parapets of box and hornbeam, a lone figure poised in a first-floor window.

Alethea had reached the laboratory after all. I climbed on to the bench's cracked seat and saw her throw the casement wide and gesture wildly. I glimpsed her for only a second, because no sooner had the panes flashed in the sunlight-for the sun, incredibly, had now appeared-than the south wing of the house began crumbling into the trench. Timbers warped and snapped, then came the landslide of ashlar and stone that exposed the library through a haze of plaster dust before its timbers likewise buckled, shedding scores of books into the great chasm. The first floor overhung the cavity for a few seconds before it began its own ponderous slide. A section of the roof lurched forward, shedding tiles; then the corbels shattered and the last of the roof spilled into the river surging through the foundations.

I was still perched on the bench, frozen with fear as the dreadful spectacle unfolded before me. I heard another scream as the east elevation fissured and collapsed like a rock-face, raising clouds of dust that billowed and swirled like cannon-smoke. The magnificent structure with its exposed compartments-each with its furniture and wallpaper-now looked no more than a doll's house or an architect's model. I could even see the laboratory with its telescope and the shelves of shattered vials. But there was no sign of Alethea, not there or anywhere else. I had leapt from the bench and was moving back through the maze when the floor of the atrium disintegrated and the doll's house crumpled inward, its floors collapsing together with a rumble I could feel in my diaphragm. I thought I heard yet another scream, but I must have been mistaken: it was only the sound of tortured iron and splintering beams, the last fragments of Pontifex Hall tumbling into the voracious water.