The disaster happened quickly, as disasters love to do.

Nero swung off to the right. There was a packman's donkey tethered outside a dosshouse, a strong young male with sleek ears and a pert backside: Nero had spotted the grand passion of his life.

As he turned he rammed the cart hard against a pastry cook's portico. The weight of lead held us fast, so he broke free. The vibrations from his joyful bellow brought down four rows of roof tiles. Stoneware went flying under his hooves as he ditched us and skittered through some potter's produce with that special dainty, high-stepping tread of a bull on the loose, all set to swerve on the spot with a horn at the ready if approached. The parts of him that were supposed to be deactivated were swinging heavily, with perilous implications for the donkey.

Women burst onto first-floor balconies. In colonnades at street level little children squealed in terror then stopped, fascinated by the scene. I grabbed the rope we kept for winding round the ox's horns and bounded after him, reaching Nero just as he reared and dropped onto his new friend. Young Ned wheezed, and squealed rape. Some misguided cookshop boy caught hold of Nero's tail. Next minute all the breath was smashed out of me as a thousand pounds of copulating ox swung round to free his rear end and sideswiped me against the dosshouse wall.

The wall, which was made of cheap rubble in a wicker frame, sagged under me enough to prevent broken bones.

I rebounded out of the house wall in a shower of stucco and dust. By now Larius was darting about on the sidelines, squeaking useless advice. What I really needed was a harbour crane. I would have run away to hide, but one fifth of this maniacal bovine belonged to Petronius Longus, my best friend.,

People were trying to rescue Neddy with anything to hand. Mostly they hit Larius and me by mistake. I walked face first into a hastily flung pail of water (or something), while my nephew took a nasty thwack from a marrow on the tender part of his neck. The donkey was trying to kick up his hind legs with some evidence of character, but once he got stuck underneath he could only brace himself for a painful surprise.

At Nero's moment of glory fortune rescued us. His victim's legs gave way (I had been frightened for his heart). Ass and ox collapsed to the ground. Ned scrambled up trembling, with a wild look in his eye. I swiftly lassooed Nero round a back leg, Larius sat on his head, and our big boy wrestled savagely beneath us-then quite suddenly gave in.

We should have been the heroes of the hour. I did expect a tussle over compensation for damaged shop fronts, and perhaps a claim under some lesser-known branch of the Augustan marriage laws, for permitting a draught animal to spike a donkey adulterously. What happened was much more interesting. The vigilante from the Decumanus Maximus had noticed us shouting at our ox with an Emperor's name. We assured him he had misheard. We called Nero 'Spot'; the fool ignored us. We called Nero 'Nero' and he ignored it too, but apparently that didn't count.

Larius and I were both arrested. For blasphemy.

The lockup for vagrants was a converted shop at the side of a temple.

'Well, this is a new one!' I chortled.

My nephew reassumed his old moody look. 'Uncle, how are you going to tell my mother I've been in jail?'

'With great difficulty, I expect.'

The jailor was an amiable duffer who shared his lunch. His name was Roscius. He had a grey spade-shaped beard and side whiskers; we gathered from his easy-going attitude that Herculaneum was the sort of inferior town which frequently arrested innocent visitors. He did keep a cellar, where he dumped anyone who looked a bit foreign, but we two had the honour of being chained to a bench where he could chat.

'Know a senator called Crispus?' I asked, mainly to impress Larius with my unflappable professional expertise.

'No, Falco.' The jailor was a man who spoke, then slowly thought about it. 'Not Aufidius Crispus? He had a house in Herculaneum; sold it to buy that boat-'

'Seen him lately?'

'No, Falco.' He thought; then opted for caution this time.

Larius felt things were unproductive. 'Show Roscius your pass!' I fetched it out; Roscius read it and handed it back.

Larius closed his eyes in desperation. I returned the pass to Roscius. 'Aha!' he said, not getting the point, but noticing that there might be one.

'Roscius, my friend, could you float that the way of a magistrate? If there's one named Aemilius Rufus, better choose him.' It still went against the grain, but whoever provided the jailor's lunch had used cold meat which turned up at the edges with a sinister dark rim. Our own relations were too far away to send provisions in. I reckoned I had about three hours before my nephew's hungry stomach gained the side effect of a very nasty attitude.

Roscius sent the pass to Helena's friend. We took turns with his flagon and all got slightly drunk.

Towards the end of a peaceful afternoon two slaves turned up to say that one of us lags had to stay locked up, but the other could come with them. I explained to Larius that he would have to be the hostage, since Rufus was the friend of a friend of mine.

'Just hurry up, will you?' Galla's treasure snarled. 'I could murder a bowl of Baian beans!'

The house of Aemilius Rufus was a modest affair, though he probably owned a stack of gracious architecture elsewhere. This one had the atmosphere of an unvisited museum. It was furnished in a heavy style with wall friezes of battle scenes and grand spiky furniture, formally arranged, which I would never dare sit down on in case I nudged a leg out of line. It was a house without the grace of children, pets, the trickle of a fountain, or growing plants. If there was a gecko on the gloomily lacquered ceilings, he kept his head well down.

His honour was on a sun terrace, which at least had the sprawling untidiness most sun terraces achieve. Its occupants had been murmuring politely, though when I shuffled out into the sunshine they seized on the excuse and stopped. After a hard day trying to stay awake in the courthouse, Rufus was relaxing at full stretch with a large goblet clasped to his chest: a hopeful sign.

He had with him a thin noblewoman who must be his sister, and another lass. They were positioned at a wicker table which held the inevitable pastry plate. The magistrate's sister picked at the sweetmeats spasmodically, while her visitor cheerfully tucked in. It was Helena Justina. She did me the supreme honour of letting my arrival put her off her food.

Unavoidable: as soon as you say goodbye for ever, you trip over the lady wherever you go. So now mine was on a Herculaneum sun terrace, licking ground almonds off her fingers, with a tantalizing smear of honey on her chin which I would have enjoyed licking off for her myself.

She was wearing white, which was how I liked her, and she stayed very quiet, which was not. She ignored me, though I refused to be demoralized by that.

The illustrious Sextus Aemilius Rufus Clemens, son of Sextus, grandson of Gaius, of the Falerna voting tribe; tribune, aedile, honorary priest of the Augustales, and currently ranking praetor, leaned around the back of his day bed; I stiffened. I was being greeted by a good-quality copy of a Praxiteles Apollo. If I stood him on a plinth with his clothes off and a thoughtful expression, Geminus would buy him like a shot. A classic face; assertive intelligence; painfully fair colouring in a rare, superb combination with extremely dark brown eyes. Helena Justina's friend was so good-looking I wanted to spit on him and see if any of the artistry washed off.