'In another time such a heresy might not have been so catastrophic. Come, Mr. Inchbold.'

We were moving almost blindly now. I could hear more rats, a whole pack of them, scampering and squealing underfoot.

'But in 1616 a war between the Catholics and Protestants was looming. Rome could ill afford new threats to its orthodoxies, especially ones propagated by someone as eminent as Galileo. Isaac Casaubon may have demolished the myth of Hermes Trismegistus, but now Hermetic philosophers all across Europe were catching at this new and, in the eyes of the Roman Curia, equally dangerous wisdom. Astronomy had replaced the learning of the Corpus hermeticum as the greatest danger to Church authority. Galileo was censured and his writings placed by the Jesuits on their Index along with the works of occultists such as Agrippa and Paracelsus. His project was dropped by the Spaniards, and the search for the longitude at sea-and for the mysterious island in the Pacific-came to an end.'

And so that might have been the last of the story, she claimed, had word not reached London that all was not lost when the Sacra Familia was wrecked on the reef. Other copies of her sea-chart existed. At first the reports were as spurious and untrustworthy as those regarding the island itself, though in time they were confirmed by spies in Madrid and Seville. These reports claimed that the Sacra Familia, after sailing from Veracruz, docked with the rest of the Mexican fleet in Havana, where, fearing the dire weather, her captain deposited duplicates of her charts, written in cipher, at the Jesuit mission of San Cristóbal-documents later shipped to Seville for safekeeping in the archives of the House of Trade.

'But that was not the only place the documents were housed. In March of 1617, just as Raleigh's fleet was preparing to sail for Guiana, Archduke Ferdinand of Styria concluded with the King of Spain a treaty under whose terms Philip recognised Ferdinand as the successor to the Emperor Matthias in return for the German territory of Alsace and two Imperial enclaves in Italy. The treaty brought together the two most powerful families in Europe, the two Houses of Habsburg, one in Spain, the other in Austria. The two great empires would now work together, uniting to share their armies and their knowledge, and in so doing to crush the Protestants of Europe once and for all. Among their most powerful arsenals, of course, were their libraries.'

A roof slate thundered overhead as it fell. Part of the ceiling had fallen to expose the beams of the garret overhead. Water was cascading through, spilling into our path. I heard a shout from somewhere behind us, then Alethea gripped my hand and pulled me through the cataract.

'But the arsenal in Vienna was in danger,' I gasped as we emerged on the other side.

'Yes. In 1617 the Protestant armies of Count Thurn were at the gates of Vienna.'

'And so the chart was taken to Bohemia?'

'Along with dozens of other treasures from the Imperial Library in Vienna. It was placed in the archives of the Spanish Rooms, which already held reams of Tycho Brahe's astronomical data as well as forbidden books by Galileo, Copernicus and other heretics.'

And so it was that the new plot unfolded in London: one that sent Sir Ambrose to Prague Castle in the entourage of the Elector Palatine. He was given the task of recovering as many of the volumes from the library of the Spanish Rooms as possible, but in particular he was charged with finding the sea-chart and bringing it to England. The decisive coup de main would be struck after all-albeit belatedly-against the King of Spain.

'But the plan miscarried,' I said. 'The palimpsest was never delivered to Lambeth Palace.'

'No,' Alethea replied. 'At the last moment Sir Ambrose betrayed the War Party.'

'Betrayed them?' We had stopped before a closed door, which Alethea was attempting to force with her shoulder. 'But why? Are you saying Sir Ambrose was a Spanish agent?'

'No, not Sir Ambrose. But both the Navy Office and Lambeth Palace had been infiltrated. Word of the palimpsest had already reached both Rome and Madrid.'

She was pressing with her shoulder at the door, which refused to budge. I heard a long-case clock chime from somewhere behind us, and then the sound of distant voices.

'Ven acquí!'

'Vayamos por otro lado!'

The door groaned and gave an inch. It was the same door, I realised, that had impeded my progress that long-ago morning. I lunged forward to help push. It creaked open another inch, then I felt a breeze and heard more frantic chiming: not spurs, as I thought at first, but the vials and cuvettes on their shelves in the laboratory.

'The fact that the palimpsest survived at all is a miracle,' Alethea said as we burst through a second later, then righted ourselves in another darkened corridor. 'In the end Sir Ambrose wanted it destroyed. Although he had risked his life to save it; his final wish was that it should burn.'

A chunk of plaster fell with a violent splash ahead of us, and the timbers above our head were creaking under an immense strain. We picked our way more cautiously through the corridor. Some more plaster collapsed, less than ten feet ahead of us.

'The Puritans wanted the chart,' I said. 'Standfast Osborne-'

'Yes,' she replied. 'As do the Spaniards. And now it appears that the new Secretary of State has also learned of its existence. Sir Ambrose claimed it was cursed, and he was right, because ten years ago he was poisoned by Spanish agents. They feared he would sell it to Cromwell, for in those years we were short of money and the Puritans were preparing for their holy war against the King of Spain. By then, of course, I knew that Sir Ambrose was not my true father,' she added in an undertone. 'That's who these men are, of course: Spanish agents. The same men who murdered Lord Marchamont.'

For a second I wondered if I had heard her aright. 'Sir Ambrose was not your father? But-'

'Yes,' she replied. 'That is my last deception. My real father was also murdered by Spanish agents-by Henry Monboddo, as a matter of fact. This was many years earlier. You see, Henry Monboddo was not only an art broker but also a Spanish agent. He learned of the palimpsest through the spies in Prague. But Sir Ambrose already knew of his treachery because of the failure of the Orinoco expedition, and he therefore used my father as a decoy. My mother, who had travelled from Prague with my father, died in childbirth shortly afterwards-'

'Your mother?'

'-and I was raised by Sir Ambrose as his daughter. I believe he regarded it as his duty, perhaps even as a form of penance, for betraying my father along with the greedy dukes and bishops in the War Party. My father was a Bohemian, a gentle man devoted to books and learning. But Sir Ambrose felt he could not trust him because he was a Roman Catholic.'

Voices echoed in the maze of corridors behind us. Alethea was moving more quickly now. We stepped over a fallen tapestry and passed a chamber whose window flashed with lightning. Through it I could see the lime trees stretching into the distance.

'Caray!'

'Por Dios! Las aquas han subido!'

The corridor turned to the left and we found ourselves splashing through a wide but empty saloon. I thought I heard a pistol shot from behind, followed by the shriek of splintering timber. Halfway through, my club foot slipped on the tiles and I sprawled headlong into the water. Within seconds I was back on my feet, hurrying, I was certain, to a horrible death.

'I was raised in Pontifex Hall,' Alethea was continuing as though oblivious to the dangers, 'and it was from Sir Ambrose that I learned all that I know. We were like Miranda and Prospero on their island, awaiting the tempest that would bring the usurpers to their shore. In time he even told me of the palimpsest and its history. He wanted it destroyed, as I have said, and I would happily have complied. But my husband and then Sir Richard each dissuaded me. The document was to be sold, you see. I would be paid £10,000. Sir Richard was acting as the agent. I had no idea who the buyer was, nor did I care. I wished to be rid of the palimpsest, that was all. I trusted Sir Richard implicitly. We were to be married. The money would have been used to restore the house. We would have lived here together.' She paused for a second. I could hear shouts coming from behind us. 'But now the usurpers have arrived,' she intoned sadly. 'And now I know what I-'