This left me with only one tactic: I would have to be nice to the bastard. I took Albia home, dumped the dog, tickled the wife and kissed the children. Julia and Favonia fell on Nux with happy squeals, though they failed to acknowledge that their father had fulfilled his promise like a hero. i told Helena I would have to miss dinner, left Albia to scare her with the explanation, and went out again.

I stomped tetchily back to the Probus Bridge, made my way past the Trigeminal Portico to the Vicus Tuscus, and climbed up to the old palace that way. I ate a bad pancake en route, which gave me indigestion; I had gobbled it, irritated at having to abandon the delights of dinner at home. By the time I reached Anacrites' office, with its unnerving smells of his clerk's discarded lunch, ink, expensive hair lotion and old antiseptic ointments, I was so overwrought at the thought of exchanging pleasantries, I was ready to sock him as I came through the door. He was out. That made me even more angry. I managed to find Momus. He carried out exercises for the spy network, but was also an old contact of mine. I liked to think he admired me, and that he thought much less of the Chief Spy. He had once been a slave-overseer, and I did wonder if in his past life he had encountered Anacrites or members of his family; I had asked that once, making a joke of it, but you don't get palace freedmen to give away much on the subject of their previous existence. They all pretend slavery never happened. They can't, or won't, remember it. I don't really blame them.

'Momus! Still working in Anacrites' filthy unit? Still slogging it out for that cretin we all despise?'

'Still here, Falco.' He gave me a look, from bleary eyes, their eyelashes stuck together with seepage from some long-term infection. His ills probably had a sexual origin, a hangover from his perks when organising slaves. Momus was big-bellied and bald, a slapdash slob who rarely went to the baths. He wore a tunic that had not been laundered for weeks and hard boots for kicking people. These days it was an empty threat; he had grown too lackadaisical to make the effort. He still yearned to torture the helpless, so just amused himself imagining pain. 'If anyone else accused me of working for Anacrites, I'd grab them so hard I'd pop their eyes out…'

There were moments I pitied Anacrites. Not only was Claudius Laeta constantly plotting to subsume the intelligence service into his own spider's web the next time secretariats were reorganised (as they were on an annual basis), but here was Momus looking on jealously, always hoping to see a big Corinthian capital fall off a column and crush the Spy, so he could inherit his post. Some of Anacrites' own field agents were light on personal loyalty as well. 'Sorry!' I said. 'You will be! What are you after?' 'Who says I'm after something, Momus?' 'You're here,' he answered. 'Given how you hate him, that's a bloody big clue, Falco! Don't tell me – you want him to release that young purple stripe he's holding?' 'Quintus Camillus Justinus, a senator's son. Well guessed. Where's the bastard put him?' 'If! knew that,' said Momus, 'I wouldn't be able to tell you, Falco.'

I could possibly disprove that statement by handing over money; Momus followed life's simple rules. 'If you really don't know, I won't bother bribing you.' 'Keep your money.' Like many corrupt men, Momus was fair. 'Well then. His office is empty. I can't even thump that pointless grubby-toed clerk he has. Save me from boiling over with frustration – I know he has a fancy house; where can I find it?'

Momus leaned back and laughed gustily. I asked him what was funny, and he said the thought of me putting on a dinner-wreath and a pleasant face to go round for an evening drink and toasted nuts with Anacrites.

I didn't have to strain my face looking friendly; Anacrites was not at home.

With directions from Momus, I had found his house. It was typical of those old, expensive places that infrequently survive on the Palatine, perfectly placed with a view of the Forum just above the Vestals' House. Once owned by names that are famous in history, these houses are now used as grace-and-favour payoffs for important officials. High walls obscured much of the view inside. The house stood on just enough land to allow carefully positioned pine trees in front of any windows people might see into. Most windows had closed shutters anyway. The property looked tended and occupied, yet it lay almost in darkness. I had the impression there would never be anyone about, no sign of household slaves even by day. But it would be well supplied with staff. Some would be for security. They would react first, and ask who you were when you regained consciousness.

I managed to force my way in through double gates and knocked heavily at the front door. An obviously enormous dog began to bark somewhere indoors. Nobody answered for a long while. Then eyes looked out through a grille and a man's voice told me the master was not at home. That was probably true. Anacrites would be so surprised someone had come to visit him, he would have me dragged indoors at once.

I contemplated lurking in a gateway opposite until the Spy was brought home in a litter, then jumping out and giving him a nasty shock as he fumbled with his door-key, but it was a cold night. For all I knew he had a woman somewhere and would stay out with her. More likely, he would wind up back in his office, brooding alone though his return there could be at any hour. Now, he could be enjoying himself at an imperial banquet; he pretended to be unobtrusive, but he liked to socialise. The thought of him nibbling snacks somewhere warm and hospitable while I tramped the dark streets on a blind errand killed my best intentions. I lost the heart to persist.

I had made an effort; before I left the Palace, I had left a cryptic note on his desk: 'Something to tell you – MDF.’ This might not set the Spy's pulse racing, but he would eventually turn up at an inconvenient moment to discover what I wanted; when I used to work with him, I had seen his curiosity boil over. The harder he pretended to be indifferent, the sooner he jumped up and rushed to investigate. It indicated lack of confidence in his own judgement. Some of us can toss an annoying note in the rubbish pail, then forget it.

No chance of that for Anacrites: Momus would also make sure he knew I had been there, and would delight in being mysterious. Anacrites always thought Momus had been put on the same corridor so he could report on him to his superiors, or to watch him for Claudius Laeta. Momus encouraged this fear by giving himself increasingly dark titles such as Inspector of Audit Inspectors. (This also upset Internal Audit, a body that assumed inflated rights and privileges under Vespasian, whose middle-class father had been a tax inspector.) Everyone but me failed to notice the salient fact: Momus was a lazy hound, whose sole aim as a government employee was to hide from notice and do absolutely nothing.

They were all paranoid at the Palace. Knowing what I did, most of them were right. Tomorrow I would probably have too much to do; tonight there was nothing else I could achieve. From Anacrites' house, I set off for home, cursing this waste of effort and time. It was typical of the Spy to thwart me. Typical that he did it without even knowing that I was. trying to find him.

It was now late. I walked quietly, keeping to the centre of the street, checking dark entrances and looking carefully down alleys as I passed. The wintry air tingled with cold. There must be snow up in the hills; sometimes ice creeps a long way down from the Alps and along the Apennines, sheeting on the edges of the lakes. Blizzards can occasionally gust as far south as Sicily. Tonight the sky was clear, making it even colder. More light filtered down from the stars high above than from lanterns, though thin cracks of lamplight showed around the edges of ill-fitting shutters. People were quiet. We had a lull in the run-up to Saturnalia, as everybody braced themselves for the real festival. Mostly I seemed to be alone. It was too cold for burglars and street-muggers, though you can never entirely rely on that. At times I heard hurried footsteps as determined drinkers made their way to bars, or slower footfalls as they left. Family businesses that would normally show lights all evening had their folding doors pulled tight across. Furniture-makers and copper beaters had finished work early. There were very few builders' delivery carts. This was no time to discover a leaky water pipe or to lose half your roof tiles; nobody can get any work done over Saturnalia, and that isn't because frost ruins mortar. Most trades in construction had already closed down for an extended holiday. Other deliveries seemed equally slack. Instead I could hear ghastly a cappella drunks serenading themselves in wailing caupona choirs. It robbed me of any desire to stop for a drink.