‘Lie back,’ I said, ‘and open your mouth.’

He was so surprised that he obeyed. I leaned forward, sniffed his breath and examined his tongue, then I untied the strings of his shirt and laid my ear against his chest. The rhythm of his heart was steady, showing no sign of distress, but there was a sheen of sweat on his skin. I was sweating myself, for there was a great fire heaped up on the hearth. How the poor fellows in the cellars would have welcomed a tenth of those coals! But in this private cell the heat was oppressive. I laid my cloak and doublet on a chair and rolled up my sleeves.

‘When were these oysters purchased?’ I pointed to the shells.

Nobody answered. I glowered at the keeper.

‘Do you want this man to die on your hands? When?’

‘Yesterday,’ he mumbled. ‘Or the day before. I cannot call to mind.’

‘And where were they kept?’

‘Here, in Master Poley’s room. Anywhere else and some thieving wretch would have made off with them.’

Master Poley, I noted. Indeed, not a common prisoner.

‘I need one of you to hold the basin for me.’

The keeper backed away. ‘I have my duties. Simon will help you.’

Simon opened his mouth, but the keeper was gone. I could hear his footsteps hurrying away down the corridor and wondered how I would find my way out of this hellish maze. Dear God, what if we should be trapped here?

‘Hold the basin ready,’ I said. ‘I’m preparing a vomitive.’

Simon looked as if he might be sick himself, but held the basin out at arm’s length, his face turned away.

‘Not like that! Lower down. Kneel by the side of the bed.’

The sick man watched me warily as I pounded the herbs for the vomitive in the small mortar I carry with me. It is best that the oils in the herbs be freshly released, not made up in advance. I warmed the last dribble from one of the wine flasks and poured it on to the herbs, then added some strong vinegar from a phial I carried, and oil of olives. The scent, bitter but cleansing, filled my nostrils, driving away the sickroom stench. I tipped the warm mixture into the wine glass and sat down on the bed, on the opposite side from Simon, and slid my arm under the man’s shoulders, lifting him into a sitting position.

‘Now, you must drink this, and in a few minutes you will vomit the contents of your stomach. You will not like it, but it is the quickest cure.’

He sniffed the liquid and his mouth curled in distaste.

‘What is it? How do I know you are not another poisoner?

I shrugged. ‘You will have to trust me.’

There was eupatorium cannabium in the mixture, root and bark of sambucus nigra (which are violently emetic and purgative), and root of viola tricolor, salvia officinalis to soothe his throat after the acid of his stomach was voided, and the calming matricaria recutita and mentha piperita. There were other, more precious things, but we do not give away the secrets of our healing potions.

‘There has been no poisoner here but yourself and that fool of a keeper,’ I said. ‘Oysters, kept for days in this heat! Are you mad? And over-indulgence in rich food and wine! You have no one to blame but yourself, Master Poley, whoever you may be.’

He gave me a searching look that I did not like, then he did as he was told and drank the mixture.

The next half hour was not pleasant. After he had vomited three times, I gave him an enema. ‘In the case of food poisoning,’ I could hear my father’s voice saying, ‘it is best to purge the gut from above and below. It will leave the patient weak, but it is the quickest cure. Keep him warm, and nothing but spring water and dry bread for three days.’

Through all this, Simon was grimly silent, assisting me, but with his mouth pursed up as if he had chewed on bitter aloes. Neither he nor the patient referred again to my youth.

At last Poley lay back, exhausted and pale. I bathed his face and hands, and gave my instructions about his diet. There was no need to tell him to keep warm, although the raging fire had died back a little. Simon took a step towards the door, but I shook my head.

‘We will stay a little longer.’ I turned to the man. ‘How do you feel yourself now?’

‘The fierce pain has gone,’ he said.

I laid my hand on his brow.

‘You are cooler, but I think it was not fever. Too much heat in a chamber can be as dangerous as too much cold.’

‘You are as gentle as a maid, young physician. What is your name?’

‘Christoval Alvarez. I am assistant to my father, Dr Baltasar Alvarez.’

‘He has a good reputation.’

I gave him a little small ale to drink, and left more beside the bed. ‘Nothing to eat until tomorrow, and then only dry bread,’ I reminded him, ‘or I will not speak of the consequences.’ My father might advise spring water, but spring water is not easily come by in London. I remembered the sweet well water of my childhood home, and the springs that rose in the hills. In London, small ale is safer than water, and weak enough even for a sick man.

I was leaning over him one last time, lifting his eyelid to check that his eyes looked clear and healthy, when he startled me by giving me a jovial punch on the chest. I leapt back, nearly overturning the table.

‘You’re a fine physician, boy. I could eat an ox now!’

I glared at him, bundled my things into my satchel carelessly and grabbed my cloak and doublet. And left the room, in such haste that I forgot to ask for my fee.

‘I hope you know the way out of this place,’ I said breathlessly. The blow had been extraordinarily hard for a sick man.

Simon looked relieved to be out of that chamber with its smell of vomit. ‘Never fear, I know the way. But it will be difficult down below, where there are no torches. Scrivens has taken the lantern with him.’

He led the way, which was easy enough at first, back along the corridor and down the first flight of stairs, where a glimmer of light from above partly lit the way. Then came the turn in the stair and the plunge into the darkness below. I remembered how worn and uneven the stones were. A false step and we would fall headlong down the whole flight. As if he felt my fear, Simon gripped me by the arm and began to guide me down, his other hand braced against the wall.

‘Who was that man?’ I asked in a whisper, though I knew not why I whispered. I could feel him shrug in the dark, we were pressed so close together.

‘I don’t know.’ He whispered too, as though fearful of being overheard. It made us oddly intimate in the dark.

‘Not a Catholic priest, I think.’

‘No.’ He gave a faint snort of laughter. ‘Too fond of the good things in life!’

That was not what I had meant. I have known Catholic priests in my homeland who lived and dressed and dined like princes, aye, and had their women, too, and their bastards, as brazen as if they answered to no God.

‘His eyes,’ I said. ‘The way he looked at us. That was no priest.’

‘One more step,’ he said, ‘and we are at the bottom of the stair. Then to the right.’

We reached the keeper’s lodge at last. Remembering my fee, I asked him for it, for it was he who had sent for my father. He blustered that I should have had the fee from Master Poley, but Simon spoke for me and with reluctance he handed it over.

Outside the grim fortress I drew a deep breath. The cold London fog, tainted with the smoke of many fires and the sewer scent of the river, seemed pure after the smell of human misery that filled the Marshalsea. I realised that all the while we had been inside the prison, my heart had been struggling like a pigeon trapped in a chimney. Only now did it steady and slow.

It had grown nearly dark while we were inside, but there was still cock-fighting nearby, for I could hear the shriek of a wounded bird, suddenly cut short as its neck was wrung. As we walked back towards the Bridge we passed the lighted windows of taverns. Inside, firelight gleamed on pewter tankards and flushed faces. And the Winchester geese were open for business, hanging out of their windows in their scanty clothes, calling to us to come in for a good time. I wondered how they could endure the cold, but then, I suppose, they would wonder how I endured my trade.