I didn't bother to enquire after Justinus, and I was not supposed to mention Veleda so I dutifully avoided that subject too. Some of you may wonder why I went to the caupona. I found no clues, searched out no helpful witnesses, turned up no bodies and announced no public appeals for informants to come forward. I accomplished nothing for the case and a pedant would argue there is no reason to describe the scene. But these are my memoirs, and I shall include damn well anything that interests me.

I was paid by results. So long as I was getting the results, my methods were my own affair. You do your job, tribune, and leave me to mine.

If it makes you feel better, let's say, a good informer who is under pressure sometimes finds it useful to take a few moments of private reflection after a busy day. 'Petronius Longus is back,' said Apollonius, as I paid up. Well, there you are. That was a result.

'What are you buying for Mother?'

Maia, the most ruthlessly organised of my sisters, was working on a list. A stylus was pushed into her dark curly hair, and her big brown eyes were glaring at a waxed tablet where various relatives' names had been assigned tasteful (but economical) gifts.

'Maia, the best thing about being married is that at last I can leave my mother's Saturnalia present to somebody else. Helena knows her duties. It saves Ma having to grit her teeth over one more manicure set that she doesn't need since five people bought one at the last minute from the same stall for her birthday.'

'Tell Helena she can do bath oils. There won't be duplicates. I had a brilliant idea – I'm clubbing together with the others to pay for an eye doctor. Galla and I are paying for the left eye operation, Junia and Allia are getting the right.' I raised an eyebrow gently. 'Discount for the pair?' 'Special one-time-only offer – two for the price of one on his low-interest instalment plan.' 'Does Ma know?' 'Of course not. She'd run off to the country. Don't you let on, Marcus. '

'Not me!' Personally, I thought another set of ear-scoop and tweezers was safer. I knew what would be involved in the cataract operation; I had investigated cures when the white scales appeared and Mother first started bumping into the furniture. I'd like to be there when my four sisters explained to Ma how she had to endure some quack pushing cataracts aside with a couching needle. The girls would probably expect me to be the heavy who held our mother down while it happened. 'In case you're wondering,' I said to Maia, 'I could use some extra weight training sessions from Glaucus at the gym.' 'You're getting a new note-tablet,' sneered Maia. I was still trying to think up ways to suggest I already owned enough notebooks to write a Greek novel, when Petro came in. He appeared to have woken from a nap, and was now gearing himself up for an evening shift on duty. This involved winding on leather wrist bands, rubbing his eyes a lot, and belching.

Petro had been out-stationed at Ostia for most of the summer, but with typical skill had wangled a move back to Rome just in time for the big festival. He and Maia, who had been living together for just over a year, were renting half a house three streets from the vigiles' Aventine patrol house. They needed plenty of room, with Maia's four growing children, Petro's daughter who was staying with them for the holiday, the cats he always allowed around the house, and young Marius' exuberant dog; Arctos had to be kept in a room away from the cats, who tyrannised him and raided his bowl. Nux, who was his mother, had gone in to see Arctos when we arrived.

Despite the way he tolerated his mangy cats, Petronius Longus had been my best mend since we were eighteen. We were both born on the Aventine, though we really met up when we knocked into each other in the recruiting queue and were jointly assigned to the Second Augustan legion. We survived our nightmare posting to Britain only by comforting each other with tall stories and drink. As we both threw up in the boat on the way over there, we already realised we had made a mistake; the subsequent horrors of the Boudiccan Rebellion only confirmed that. We got out of the army, no one needs to know how. Now he ran criminal investigations for the Fourth Cohort of vigiles, while I ran a private enquiry business. We were both damned good at what we did and we were on the same side in fighting life's filthy surprises. Now he had finally settled with Maia, after yearning after her for years, and for both their sakes, I hoped it lasted.

'Io, Marcus!' Petro thumped me on the shoulder. He enjoyed festivals. He knew I hated them. I gave him the gloomy scowl he expected.

He was taller than me, though not enough for it to matter, and broader. As a vigiles officer, he had to be. When the arsonists and other villains weren't attacking him with fists and knives, the ex-slaves he commanded were giving him almost as much trouble. He handled it. Petronius Longus could handle most things except the death of a child or an accident to a pet cat. In our time, I had seen him through both. He had stuck by me in bad situations too. 'What are you working on, Marcus?' 'I am not allowed to tell you,' I complained solemnly. 'Well, spit it out at once then, lad. I won't pass it on.' 'That a promise?' 'Same as the one you must have given somebody…' 'I gave my oath to Tiberius Claudius Laeta.' Petronius grinned broadly. 'The big poppy at the Palace? Well that's all right; it doesn't count.' Trust a public servant to take a realistic view. In a few taut sentences I summed up the mission for him.

There was a reason why I was bringing Petronius into my confidence. I explained – though to him it was perfectly obvious – that with the whole of Rome to search and no clues, I stood little chance of finding Veleda, let alone both Veleda and Justinus, aided by only a handful of lackadaisical legionaries from Germany. 'This stinks.' He sounded calm. 'Surprised?' 'It's one of your jobs, you idiot. You're going to need our help as usual.'

'It's a rat's arse,' I agreed quietly. 'In which, as you so rightly notice, it differs from my usual commissions by not one digit of linear measurement. That Veleda is on the loose in Rome, and has been for over ten days, is a State Secret of some delicacy -'

'Everyone's heard about it,' scoffed Petro. He let out another belch; he claimed this kept him fit. Maia just glowered. They were like an old married couple; although both had been previously hitched to other people, most of us thought these two should have been sharing a bed from the start.

I carried on: 'Anacrites has been put in charge of an official hunt, using the Praetorians -' This time Petronius really swore. 'Right! If the Praetorian Guard, fired up with Saturnalia drink, find Veleda, she'll become a new, and ghastly, festival game.' The vigiles would not be delicate with her either, but I left that to his imagination. Petro was well aware that his cohort was composed of roughs and toughs; in truth he was proud of them. 'And the common people are terrified of barbarians invading the citadel, so they will tear Veleda apart.'

Maia, who had been silent and apparently absorbed in her Saturnalia list, looked up and inserted in a caustic tone, 'That is nothing to what Claudia Rufina will do if she catches her.' Petronius and I both winced. 'Give me a description to circulate,' Petro offered. 'I'd like this to be kept from your tribune, you know.' 'Be realistic, Falco. Rubella needs to know – and what's more, so do his oppos: you need this to be given to all the cohort tribunes because Veleda could be anywhere. She may know that you live on the Aventine and Justinus lives by the Capena Gate, but in what? – nearly two weeks – she hasn't come looking for either you or him. So by now she could be hiding up in any of the districts – assuming she is hiding up, and not being held by some bastards against her will somewhere.' I was protesting, but he stopped me. 'I can put it forward as a game the tribunes will all like: "find the lost prisoner first, to annoy the Praetorians". They will do it, and be discreet.'