We had been told to come in informal Saturnalia dress. That still meant looking smart because the Emperor, Titus and Domitian would be present. They would patrol among us, pretending to be part of one giant family. So we had to concoct a reversed-rank version of formality, dressing up as we pretended to dress down. Most of the women had just borrowed their slaves' frocks then piled on as much jewellery as they could. The men looked uncomfortable because their wives had chosen their dinner robes and, according to recognised domestic rules, had chosen dinner robes their husbands hated. I had been put in blue. On men, blue is for floor designers and second-rate shellfish suppliers. Helena, who often wore blue and looked gorgeous, tonight was in unaccustomed brown, with rows of crimped hair that must have taken all afternoon to set. Unless it was a wig; I did wonder. She looked like a stranger. The ridged hair had added five years to her and seemed to belong to some impoverished orator's parchment skinned spinster sister. 'That's certainly a disguise.' 'Don't you like it?' 'I'll like you better when you take it off,' I affirmed salaciously. If you are going to abandon a mission for an evening, you may as well get into the festive spirit and while you are at it, try to seduce a girl. Helena reddened, so I reckoned I was in there.

Camillus Verus was wearing his normal white, complete with full senatorial purple stripes. 'Olympus, I'm overdressed for this fiasco, Falco!' Nobody had reminded him that he had to play the part of a slave tonight, and somehow he had omitted to consult on his outfit with his wife. Julia Justa must have been preoccupied; she was having problems remaining decent. She had decided that playing lowborn and low budget must mean wearing low-cut. Inexperienced at flaunting, she kept fiddling with the skimpy drape across her bosom. Her husband tried to stare in other directions, pretending not to notice her difficulties. He was terrified she would ask him to assist with pinning things.

'And who have you come as, Marcus dear?' chirped Julia, bright with embarrassment. Her discomfiture inevitably drew the eyes directly to its cause.

I must have been wearing the horrified rictus of any man in danger of glimpsing his mother-in-law's nipples. 'I think I'm a back-alley debt-collector. ' 'Isn't that rather similar to what you do normally?' 'I don't work in a damned skyblue tunic!' 'Indigo,' murmured Helena. 'I feel like a periwinkle.' 'Be good. It will soon be over.' Helena was fooling. It took us almost an hour merely to obtain seats. You needed to be fit. If there had ever been a table plan, nobody could find it. We squeezed in only by shoving harder than the people who were trying to climb on to benches ahead of us. 'As soon as the first course is over, everyone can get out their cloaks. Then it won't matter what you look like.' We did all have cloaks. We needed them too, dining under the stars on a gusty night in the middle of December. To do Saturnalia properly means celebrating the new crops in the wide outdoors. Helena and I were both longing for a warm brazier indoors and two comfortable armed chairs, each with a good scroll to read.

Near the temple steps, adjacent to Saturn's awesome spread, was a table for the imperial family and their courtiers. King for the Day was a public slave, but he had been carefully chosen – an elderly palace scribe who could be trusted to behave sedately. His mischief-making was forced: he kept eyeing the chamberlains to make sure he had not gone too far. 'He's a bummer. I think I ought to help him out -' That was not me, but the senator. 'You stay where you are!' commanded his wife.

Once I had thought this couple staid, but the more I knew them the more I could see where their three children had acquired eccentricity and humour. There was the senator, winking wickedly at Helena as if she was still a giggly four-year-old. Here was Julia Justa, that rigid pillar of the cult of the Good Goddess, showing more cleavage than a cheap whore in a travellers' inn; what's more, just like Ma, she distrusted food at public banquets and had lugged a hamper here. The only difference was that Julia's home fare had been cooked and packed by a battalion of slaves.

It caused a problem for me. Men of action eat or work. It is bad practice to attempt both simultaneously before a busy night. My physical trainer would have been horrified to see Julia Justa's tempting nibbles and nuggets find their way into the cheap food bowl we had all been provided with.

Vespasian, our untroubled old ruler, tossed away his wreath happily when he progressed to his place at table. He looked jovial, but I noticed he managed to avoid any real indignities. His staff played the festive game by bowling the occasional apple at one another, making quite sure none hit the Father of his People. I recognised Claudius Laeta, plus a couple of other palace retainers I knew, and a man in a discreet moleskin-coloured tunic, who had his back to me but who could only be Anacrites. A small group of Praetorian Guards, bareheaded to suggest informality, were lounging at the back of Saturn on the temple steps; they may have shed their glittering crested helmets, but they were on duty to protect the Emperor.

Titus and Domitian, Vespasian's chubby sons, made themselves amiable by moving around the tables and sitting with ordinary folk. They both wore plain tunics, but in purple, so it was obvious they were princes being gracious. I saw Titus laughing and joking diligently, some distance from us. Domitian was working our sector of the crowd, but came no nearer than the end of our table, still out of earshot. He and I loathed each other, but I was confident he would never start anything with his father or elder brother watching.

As the noise of participants rose until it almost drowned the music of a few polite tambourinists and flautists, I busied myself attempting to acquire some of the thimble-sized cups of wine. The senator was talking to a neighbouring diner, so he could ignore the fact that his wife kept diving under the table to extract dainties from her hamper for us all. Every time she bobbed up again with new treats hidden in her dinner napkin, her dress had slipped even lower. I rather suspected the noble Julia had been plied with tots of false courage while her wardrobe mistress and makeup girl were decking her out for this occasion. Maybe the old republicans were right and it was shameful for women to drink. Meanwhile Helena Justina, that model of moral rectitude, grabbed a tot, knocked it back, pulled a face, and snaffled another one.

A sewer rat ran across the table. He thought the Forum belonged to him at night. I was the only one who noticed. Everyone else was screaming with laughter at the antics of a group of professional entertainers who were dressed as circus animals. I had never seen so many fake woollen manes or such thickly plastered artificial hide. They were rather warty. Some were going to lose a lot of skin when they tried removing their rhinoceros masks tomorrow. One frolicking jester tried investigating Julia Justa's cleavage; he got his horn stuck on her pearl necklace, without doubt purposely. 'Aah… Decimus, help me!'

Now I was happy. It was worth coming, to have seen my father-in-law removing a clown from his wife's naked bosom by applying the fulcrum principle to the fellow's rhino horn. The appendage had been well glued on. The man's screams must have sounded right up on the Arx.

It was Helena, standing up so she could more easily reorganise her mother's disarray, who spotted another flurry of excitement. 'Marcus! Someone you know has had an accident…'

I followed her gesture. Behind the statue of Saturn a man had fallen over awkwardly on the spilled oil. It was Anacrites. Like me, he must be waiting his moment to slip away from the banquet unobtrusively; I thought I could see slaves with a litter waiting in the narrow side street by the temple. He must have tried to disengage from the courtiers' table and sneak around behind the statue, but when his foot skidded under him, he crashed against the image of Saturn and nearly pushed the god right over into his golden bowls of ambrosia. Fortunately the statue was held in position by hidden wooden bracing. As Anacrites stumbled back on to his feet, concerned slaves rushed to help him – which was what had attracted Helena's notice. They were anxiously checking that Saturn was still safe, under cover of testing if the Spy had a twisted ankle. I wished it was his neck he had twisted.