She pulled back from him. 'Wait, I have to set the alarm system. I'm the last one out.'

'You're lying to me, Amy.' He bared his teeth and yanked her arm hard. 'Don't try to trick me. I can see inside your head.'

They passed from the lab along a corridor, and on to a broad staircase. Miles should have come for her by now, but they passed no-one, not even Dr Harold Masters, who was usually making tea in the cubby-hole beside the staircase at this time of the evening.

Spanky's gripping hand felt as though it was burning into her wrist. At the main entrance, the two security guards barely looked up from their desks to wish her goodnight. Couldn't they see that she was in trouble?

The rain sizzled against Spanky's back as he strode across the museum Forecourt with her. Amy maintained her grip on the casket, frightened that she would be punished if she tried to fling it away. 'Where are we going?' she gasped, frantically trying to keep up with him.

'To the departure point,' he snapped, barely bothering with her. He crossed Museum Street, half dragging her upright as she slipped on the wet tarmac. He moved so quickly that she found herself being bodily lifted by him at moments when the traffic seemed about to crash into them. Onward they moved, through Holborn and down toward the Embankment.

They were standing in the centre of Waterloo Bridge with the great rain-swollen river sweeping beneath them, broadening out on its way to the sea. 'Why here?' she shouted, the roar of wind and traffic filling her ears.

'I need a good run-up,' he replied. 'Got a tight grip on the casket?'

He checked the box pressed against her sodden breast, then produced an old-fashioned cut-throat razor from his coat and passed it to her with his free hand. 'Hold this. I'm letting go of you for a moment. If you try to escape I will kill you, Amy, I think you know that.' Spanky tore off his jacket and shirt, throwing them out into the Thames.

'I want you to take the razor and run it along my spine.' He pointed to his broad rain-spattered back. 'Do it quickly.' He snapped open the blade for her.

Shaking with cold and fear, she suppressed a shudder of horror as she touched the blade to the point he indicated between his shoulderblades.

'You'll have to push harder than that. Pull it straight down. As deep as you can.'

Wincing, she did as she was told, pushing on the blade and dragging it down. The edge sliced smoothly and cleanly as the skin of his back opened in a widening crimson slit. Spanky was drawing breath in low, guttural gasps, part in pain, part in the pleasure of release from his confinement. As the blade reached his trouser-belt he slapped it from her hand. The razor skittered across the pavement and slid into the gutter. Swathes of blood washed across his back, diluting in the downpour.

Spanky bent forward with an agonised shout and the epidermis split further apart across his back. From within the carapace of skin, two enormous black wings unfolded like opening umbrellas. As the joints clicked and cracked, the membranes between them flexed and stretched and grew. At first she thought they were made of black leather, but now she saw that they were composed of thousands of tiny interlocking black feathers. He seized her hand and climbed on to the balustrade of the bridge, dragging her up on the ledge with him. The fully opened wings spanned a distance of eighteen feet above them.

'Hold on to your hat.' He turned and gave her a mad grin. 'Here we go.'

Amy's stomach dropped as they launched from the bridge. Intoxicated with the terror of his unbreakable grip, she screamed and howled into the racing clouds above. They swooped down to the scudding grey water, almost touching their shadow, then up and along the path of the river, moving so fast that they outdistanced the falling rain. The pain in her clutched wrist was excruciating. He turned and brought his face close to hers, shouting as the great black wings beat powerfully above them.

'You have the casket.'

'Yes,' she shouted back as they started to climb, 'I have the casket.'

'Then we can make the crossing.' He pumped his membraneous wings fasted ever faster, so that they flexed and shook from humerus to metacarpal, and it seemed that they were moving beyond the speed of earth and sea and weather and light and time.

Something bright shone in her eyes. She forced herself to look up. Ahead in the clouds, a dazzling area of light had cleared the grey rain to send a mandelbrot set of fractal colours spiralling down toward them, like pieces of rainbow glass from an exploded kaleidoscope.

'You see it?' he bellowed. 'You see it? That's where we're going. Inside there.'

'No!' she screamed, knowing instinctively that the experience would kill her instantly. This was not a sight for mortal eyes. But they were racing forward at such a velocity that nothing could stop them from reaching the area now. Piercing shards of diamond brilliance enveloped them as they left the earth behind forever and plunged into the heart of the world's existence.

And just as they reached it – it was gone. Slammed shut, vanished, the colours all disappeared, nothing ahead except endless cold grey sky.

Spanky's face was contorted in fury and terror.

'The rings of Cain!' he yelled at the heavens. 'I am returning with the rings!' Already his wings were parting with the impossible velocity, flesh and feathers tearing off in strips, revealing birdlike bloody bones beneath.

With nothing to propel them, their speed slowed. For a moment, it seemed that they were hanging in the air. 'You have the rings,' he screamed at her.

'No, I told you – I have the casket.' The box was still unlocked. She had emptied the rings out as they flew. He had not noticed. With all his energy and concentration centred elsewhere, he had not seen the seven iron bands scatter in the wind and fall back toward the river, and now the doorway home was closed once more.

A sharp crack resounded above them as the great wings bloodily shattered and folded, and with a sickening lurch they dropped back toward the earth. Spanky's anguished howling filled her tortured ears every metre of the way.

Down and down.

The glutinous silt of the river formed undulations across the expanding estuary at Dartford. It trapped all manner of debris swept out with the heavy ebb tide. It cradled Amy's unconscious body, rolling her gently against the shore until some kind old souls spotted her, and dragged her out to warmth and safety. Inside Amy's jacket they found an old casket, gripped so tightly that the corners had bruised her flesh.

Spanky's broken form had fallen more heavily and plunged much deeper, to be snagged by the twisted metal on the riverbed. Held firmly in place, Chad Morrison's body undulated against the current. His earthly form was dead, from the fall, from the loss of blood, but the daemon was still alive and imprisoned within. There was nothing Spanky could do but stare out from his blanched shell in endless horror, gripped by his prison of bloating dead flesh, held in turn by the detritus of the river, beneath that great protector of the city.

He was aware of everything, and unable to do anything. He even thought he saw one of the precious rings float by, inches from his eyes. Eventually he allowed his senses to dull and close, lulled to a dreamless sleep by the lunar tides.

Somewhere inside the wide pulsing currents of the sea, the seven rings of Cain tumbled and drifted, lost to man and lost to angels.

'And that is how Karl Fabergé's most magnificent casket, so beautifully restored by Amy Dale, came to be exhibited here at the British Museum,' said Dr Harold Masters, eyeing his bored students as they sprawled and drifted in various states of semi-consciousness about the lecture room like dumped shop mannequins. Honestly, he thought, you try to bring history alive for the young, but you might as well not bloody bother.