Following me home put his presence in a different light. That degree of curiosity could not be innocent. It meant his interest in the warehouse was no coincidence. He must be some character who badly needed to know what had happened there, and to put a name to any stranger connected with the place. This spelt trouble for those of us at the Palace who were thinking we had put the conspiracy against the Emperor to bed.

While I watched, he lost his zest for spying and took himself off strolling towards the Ostia Road. I had to find out more about him. I raised an arm to Lenia, left Secunda with a smile that ought to keep her simmering, then slipped out in pursuit.

Cassius the baker, who must have been keeping an eye on the stranger, tossed me a thoughtful look and a stale bread roll as I went by.

I nearly lost him on the main road. I glimpsed my mother inspecting onions by a greengrocer's stall. Judging by her grim face, the onions, like most of my lady friends, fell short of her standards. My mother had convinced herself my new job working for the Palace involved good money, simple clerical work, and keeping my tunics clean. I was reluctant to let her discover so soon that it was the same old round of trudging after villains who chose to slouch through the streets when I wanted my lunch.

It took deft footwork to avoid her without losing him. Luckily the green cloak was an unpleasant shade of viridian, easy to pick up again.

I trailed him to the river, which he crossed on the Sublician Bridge; a ten-minute hike away from civilization to the Transtiberina hovels where the street hawkers crowd when they are kicked out of the Forum after dark. The Fourteenth District had been part of Rome since my grandparents' day, but there were enough swarthy immigrants over there to make it feel alien. After my task that morning I did not care if one of them knifed me in the back.

When you don't care, they never try.

We were walking in deep shadow now, through streets overhung by perilous balconies. Thin dogs ran in the gutters. Ragged, lug-eared gypsy children yelled at the terrified dogs. If I let myself think about it, the whole district terrified me.

The green cloak was travelling at a steady pace like a citizen going home for lunch. His build was ordinary, with thin shoulders and a youngish walk. I still had not seen his face; that hood stayed up despite the heat. He was too shy to be honest, that was sure.

Although I kept water carriers and piemen between us out of professional etiquette, there was no need. He did none of the ducking and dodging that would have been sensible in this seedy neighbourhood. He never once glanced behind.

I did. Regularly. No one appeared to be shadowing me.

Above our heads limp blankets aired on ropes, and below that other ropes carried baskets, brassware, cheap clothing and rag rugs. The Africans and Arabs selling them seemed to accept him, but exclaimed to each other harshly as I passed; still, they may have been just admiring me for a handsome lad. I caught the scent of new flat bread and sickly foreign cakes. Behind half-open shutters worn women with ugly voices were shouting their exasperation at idle men; occasionally the men lashed out in temper, so I listened with fellow feeling while I quickened my step. In this area they sold intricate little copper knives with spells traced on the blade, addictive drugs distilled from oriental flowers, or boys and girls with limbs like cherubs whose trade in vice was already rotting them with hidden disease. You could purchase the promise of a heart's desire, or a shabby death-for someone else, or for yourself. If you stood on one spot too long, death or some worse crisis might find you without the bother of praying for it.

I lost him south of the Via Aurelia, in an ominously silent street, about five minutes into the Fourteenth.

He had turned up a narrow alley, still marching at that regular pace, and by the time I made the corner there was simply no sign. The place had unhappy doorways let into sheer grey walls, though it was probably not so sinister as it seemed.

I wondered what to do. There were no colonnades to lurk in, and my green friend's siesta might last all afternoon. I had no idea who he was or why we were tailing each other. I was not sure I cared. It was the hottest part of the day, and I was losing interest. If anyone in the Transtiberina suspected I was an informer, I would be found on the pavement tomorrow with some criminal's monogram carved on my chest.

I noticed a sign for a drinking shop, entered its cool gloom, and when the squat-necked, big-bosomed woman who ran it dragged her hulk into view I ordered spiced wine. No one else was there. The shop was tiny. There was one table. The counter was almost hidden in the dark. I felt the bench for splinters then sat down cautiously. It was a place where a drink took ages to come, because even for a foreigner the madam simmered it hot and fresh. This natural hospitality left me churlish, and caught off guard; both feelings were all too familiar.

The woman disappeared again so I sat over my beaker alone.

I laced my fingers together and thought about life. I was too tired to manage life in general, so I concentrated on my own. I rapidly reached the conclusion it was not worth the denarius it had cost me to sit here pondering with my wine.

I felt deeply depressed. My work was dreadful and the pay worse. In addition to that, I was just facing up to ending an affair with a young woman I hardly knew yet and did not want to lose. Her name was Helena Justina. She was a senator's daughter, so seeing me was not quite illegal, though scandalous enough if her friends found out. It was one of those disasters you start knowing it must be hopeless, then end almost immediately because continuing becomes even more painful than breaking off.

I had no idea what to say to her now. She was a wonderful girl. The faith she had in me filled me with despair. Yet she probably saw that I was sliding away from her. And knowing she already understood the situation was not helping me compose my leaving speech…

Trying to forget, I swallowed blankly. But the catch of hot cinnamon on the roof of my mouth recalled that warehouse earlier. Suddenly my tongue felt like gravel. I abandoned my wine cup, clinked some coins onto a plate and called goodbye. I was on my way out when a voice behind cried 'Thanks!' After I glanced behind me I lingered after all.

'Don't mention it, sweetheart! Has the woman I saw first been practising witchcraft, or are you someone else?'

'I'm her daughter!' she laughed.

You could see (just about) that she was. In twenty years' time this gorgeous little body might look just as unappealing as her mama-but she would be passing through some fascinating stages on the way. She was about nineteen now, and this was a stage I liked. The wine-shop woman's daughter was taller than her mother, which made her movements more graceful; she had huge dark eyes and tiny white teeth, clear skin, tinsel ear-rings, and an air of perfect innocence that was flagrantly false.

'I'm Tullia,' this vivid vision said.

'Hallo Tullia!' I exclaimed.

Tullia smiled at me. She was a cuddlesome armful with nothing much to do that day, while I was a man whose low spirits needed consoling. I smiled back at her gently. If I had to lose the sweet lady I wanted, unprincipled women were welcome to do their worst with me.

A private informer who knows what he is doing can soon make a barmaid his friend. I engaged her in harmless banter then eventually broached, 'I'm looking for someone; you may have seen him-he often wears a cloak in a rather evil shade of green.'