‘You lock your houses in Duck Lane?’ His tone held a touch of contempt, understandably, here where the butchery stench filled our lungs and straw stained with blood and animal dung blew about our knees.

‘Some of our medicines are dangerous in the wrong hands. They must be kept safe from curious fingers.’

We turned away and began to make our way towards the river, leaning against the wind, the boy urging me along, though I would not run on the icy streets – frozen mud at first, cobbles slippery as eels when we reached Cheapside.

‘What is your name?’ he asked, as we neared London Bridge.

‘Christoval Alvarez.’ I did not ask his. We do not become intimate with strangers, we who are known as Strangers ourselves.

‘I am Simon Hetherington,’ he offered, as if wishing to make up for his earlier discourtesy. ‘You are Spanish?’

‘Portuguese.’

‘Ah.’ He slid his eyes sideways towards me. ‘A Portingall. I see.’

‘We are the best physicians in London,’ I said, unwisely, stung by his knowing tone. Then I ignored him.

Halfway across the Bridge he stopped suddenly, doubled up again with pain.

‘What ails you?’

‘I must have run too far, too fast. I feel as if someone were stabbing me in the guts with a knife.’

‘Perhaps you are poisoned too,’ I said unkindly. ‘Stop here, where there is a break in the houses. You can lean against the parapet. Where does it hurt?’

He did as he was told and pointed mutely at his right side, about the level of his waist. I took off my gloves and began to massage his side, reaching up under his doublet. After a while I felt the knotted muscles relax and heard him give a sigh of relief. Then I handed him a rolled pellet from my satchel.

‘What is this?’

‘Stellaria holostea.’

‘What?’

‘Stitchwort. It will ease the pain, though it is better steeped in hot liquid.’

He eyed it for a moment, then put it in his mouth and swallowed it with a grimace.

‘For all your sharp tongue, you have a kind touch in those fine hands of yours, Christoval.’

I turned aside and hastily pulled on my gloves, then I leaned on the parapet beside him.

‘Most people call me Kit. Christoval is a name for feast days and ceremony.’

‘Kit, then.’

We were facing down river, where the Tower rose fearfully on the left, behind the tangled masts and spars of great sea-going ships jostling for anchor room near the legal quays and the Customs House, where I sometimes went on errands for Dr Nuñez or Dr Lopez or my father, who all had an interest in the Portuguese spice trade as well as in medicine. There must have been fifty or sixty ships moored, even at this winter season. One high-pooped merchantman was just setting out on the evening tide, heading towards Gravesend and the open sea. Would she be making for my homeland? Or perhaps for the Low Countries, where our people had relatives living in Antwerp and Amsterdam? On the south side of the river, clustered fishing boats made towards the Deptford shipyards to unload their catch for the royal household at Greenwich. Against the slate sky, the ravening gulls sliced down towards the boats like white scimitars, their wings catching the last of the light from up river, so that they swung from dove grey to silver to white, and as the sun bled down below Westminster the scimitars were encarnadined as if with blood. I shivered. It seemed a bad omen.

We trudged on over the Bridge and under the rotting heads that decorated the spikes over the Great Stone Gateway. When we had first arrived in London, four years before, I refused to pass that fearsome place alone. I used to dream repeatedly that one of the heads toppled off and fell on me. I would wake screaming, sweating with fear, until my father held me and crooned me into quietness. Now I never looked up, but I will not say that I was unaware of them.

A short way along Bankside, near the church of St George, we came to the Marshalsea, a towering grey wall surrounding it, crowned with iron thorns, blackened with London’s sooty smoke, and somehow greasy, oozing a foul stench and dirt of its own, like some diseased and rotting body past hope of any cure. Hell in Epitome, it was called. I had never been inside, but Simon knocked confidently on a low-browed door in a kind of lodge bulging out from one of the corner towers like a carbuncle. He exchanged a few words with someone inside, and we were beckoned in.

It had been growing darker outside, but across the barren yard and within the arch of the prison doorway itself the darkness felt tangible, like a bag over the head, pressing against my face and robbing my lungs of air. I stopped. My palms were sweating. My feet refused to move forward over that fearful threshold. There was a stench of urine and faeces and rot and mould and sickness and despair. Outside, the prison had loomed up as a silent presence in this area of gaiety and entertainment. For all around were the noisy brothels and alehouses and bear-gardens, the shouts at the cock-fighting and laughter at the playhouses, while the Marshalsea stood frozen in its formidable silence. Once inside the thick walls, however, we could hear the real voice of the Marshalsea.

Simon tugged impatiently at my elbow as the keeper (for I supposed that was what he was) emerged from his cubbyhole, and I stumbled inside. The narrow corridors along which we hurried boomed with a continuous moaning, like the wind over the stormy sea when our ship was crossing the Bay of Biscay. From behind the thick stone walls and the implacable oaken doors, the cries and prayers lost all meaning, blending together in that terrible wail of human misery. It is our calling to minister to all who need us, even the poor and impotent, but I would have been glad, at that moment, to have fled almost anywhere else, except back to Portugal.

The keeper led the way with a rusty candle-lamp and at length began to ascend some stone stairs, so dipped at the centre that they showed the footsteps of centuries. Simon hurried to keep up, while I trailed reluctantly behind. After two flights of stairs, the corridor opened out, the doors were more widely spaced, and there were torches burning in sconces on the wall. This must be where the better class of prisoner was kept, those who could afford a room to themselves, furnished in some comfort, and who could send out for meals and wine. I should have realised. Simon would not have been sent running all the way across London, and beyond the city wall to Smithfield, for the sake of some poor wretched creature in the common crowded cells below.

At the end of the corridor a door stood wide, allowing light to spill out, overwhelming with its abundance the keeper’s poor lantern. He blew out the candle and ushered us into the room. It was only then that he looked me full in the face. At the same time the man in the bed, struggling up from his pillows, glared from the keeper to Simon, then fixed on me a look of scorn.

‘You were sent to fetch a physician, boy. Who is this?’

I ignored him and set down my satchel and gloves on a table strewn with the remains of a meal: a fine pewter plate smeared with rich gravy, a pile of oyster shells, a half-eaten bowl of some sticky pudding, from which cream dribbled on to the table, turning yellow as mustard as it soured. There were two nearly empty wine flagons of thick green glass, the kind in which only the finest quality of wine is sold. One drinking glass. And an untouched leather flask of small ale. I turned my back to the men, so that they could not see how my hands shook as I unfastened the buckles of my satchel. At least the door of the cell was left open, else I do not think I could have drawn breath.

‘This is a boy, not a physician.’

The man on the bed would have been handsome, had he not been sticky with fever and spattered with vomit. Perhaps in his thirties, with heavy rings on his fingers and the look of good living, even self-indulgence, in the smooth cheeks above the neatly trimmed beard. He had thrown aside his doublet and shoes, but otherwise was fully dressed. On the floor beside the bed was a basin of vomit, not much, thin and yellowish. I smelled it.