`What's this?' cried Tibullinus, sounding annoyed, though he tried to hide it. `Checking up? On the Sixth?'

`Far be it from me to slander- the meticulous Sixth!', Petro assured him. He was a devious bastard when he chose. 'Just a couple of lads I told to lend a hand when they'd finished something else. Looks like they only just caught up with you.

Everyone realised his couple of lads had attached themselves to the Sixth and their not-quite prisoner for the whole journey – and that the men of the Sixth had failed to notice they were being tagged. They should have known. It could have been any kind of ambush. We left it at that, before things became too sensitive.

Something was about to happen.

There was a moment's unnatural atmosphere, then everyone straightened and grew watchful. The carriage door creaked as it opened. Then Balbinus emerged.

ALWAYS THE SAME shock: you come face to face with a murderous master criminal, and he looks like a ribbon-seller. Balbinus Pius was five feet three digits – definitely not tall. He was looking me in the windpipe, and appeared not to notice that most of the officers present overstripped him by almost a foot. He had an oval head; an expressionless face; wavering eyes; an anxious expression that verged nicely on bewilderment. His manner was quiet; no more threatening than a ladybird.

His hunched shoulders held up a dapper white tunic and short grey cloak. The cloak was pinned extremely neatly on the left shoulder by a round gold brooch set with five garnets. He had healthy pink skin. On the top of his head it was visible through the short, thinning down of near-baldness; the bushier stuff above his ears had been lathered with some discreetly piquant lotion. He wore dark grey leather travelling boots. His seal ring was gold, a Greek design of a winged female driving a four-horse chariot. He wore two others for ornament, one set with sapphires and opals, the other openwork, cut from sheet gold with added granulation. He wore the plain wide gold band of the middle rank. He carried no weapons..

I was annoyed, and so was Petro, that Tibullinus, Arica and some of the other men of the Sixth stepped forwards and shook hands with him, bidding farewell. Words were exchanged. Unable to tolerate it, the rest of us looked away and breathed disapproval. We were reluctant to become part of the conversation. We were resisting being coerced. We had glimpsed the complacency amidst which corruption flowers. °

`How can you do that?' Martinus spluttered at Arica; Arica had actually slapped Balbinus on the back, as if he were seeing off his own cousin to the army. Martinus always spoke his mind.

`No harm being polite.' The Sixth had been supervising Balbinus' movements ever since he went to trial. Contact would have been unavoidable.

The whole group of the Sixth began standing back now that they had delivered the package to us. As soon as he saw them shaking hands with the criminal, Petronius Longus had abandoned any pretence that this was a joint mission. His normal easygoing manner had vanished; I had never seen him so serious. The rest of the climax belonged to him and to the Fourth. Once the Sixth had formally taken their leave, they slunk from the scene.

I said nothing, but I had a sense that Petro's night of triumph had just been spoiled.

The freedmen had taken all the luggage on to the ship. They stayed aboard. We could see sailors assuming their places at the mooring ropes. The captain hovered at the head of the gangplank, impatient to sail now he had the breeze and approaching light. None of us made any attempt to look for Linus. It was best to forget he was there.

The vessel was a roomy merchantman called the Aphrodite. Balbinus would be well set up; there was a cabin for the captain and favoured passengers; a latrine hanging over the stern, even a galley where food could be prepared. The Aphrodite was half as big again as the ship on which Helena and I had returned from Syria. She needed to be strongly built to make such a long voyage so late in the year.

Now the criminal stood looking hesitant; he seemed uncertain what was expected of him. `Am I to board?'

His doubt did not last. Petronius Longus appeared in front of him, flanked by Martinus and me. The other squad members clustered close, in a tight circle.

`Just a few formalities.' It was clear that now Balbinus was in the care of the Fourth Cohort there would be no hail-fellow handshaking. `I've waited a long time, Balbinus,' Petro said.

`No doubt you have done your duty, officer.' The man spoke with reproach. He still seemed like a tunic-braid salesman – one who had just been told to his amazement that his embroidered Egyptian fancies had leaked crimson dye all over ten togas at some swanky laundry. `I am innocent of the crimes of which I have been accused.'

`They all say that,' Petronius complained, addressing the sky in despair. `Gods, I hate this hypocrisy! A straight villain always respects a straight arrest. He'll shrug and accept that he's caught. But all you self-justifying types have to make out that you cannot believe anyone could so terribly misjudge you. You convince yourselves all that matters in a civilised society is for men like you to continue your businesses without interference from officious sods like us. Sods who don't understand.' Petronius set his jaw so hard I thought I heard his molars crunch. 'Only I do understand!' he sneered. `I understand what you are all too well.'

This rant had had no effect. Balbinus' eyes, some colour you wouldn't bother to notice, wandered to me. He seemed to realise I was an outsider, and was hoping for some sympathy. `You had your chance,' I told him, before he could start whining. `The benefit of a jury trial, in the calm of the Basilica. Six lawyers. A jury of your equals, who heard about your activities without allowing themselves to be sickened. A judge who, even while passing sentence, was polite. Meanwhile outside, market traders still had their takings grabbed by your rampaging street gangs. Near destitute old women were being tricked out of their savings. Men who dared to resist your hold-up thieves spilled their lifeblood into the gutter. Female slaves were sold into prostitution by angry mistresses after your footpads snatched the shopping money -' Petronius moved slightly. I fell silent.

`Is there anything further you wish to tell me about your business?' Petro's request was formal; a vain hope.

`I am innocent,' Balbinus intoned solemnly.

Petro's sarcasm was milder than I expected: `Oh, for a moment I thought you were going to surprise me and admit something.'

His men were on edge, wanting to retaliate, wanting something to make them feel good.

Petronius held out his hand, palm upwards. `You can keep what you stand up in. I need your equestrian ring.'

With automatic obedience, the big rissole pulled off the badge of his lost social status, struggling to wrench it over his first knuckle bone. He looked puzzled again. `May I have a receipt?'

'No need.' Petro took the small band of gold between finger and thumb as if it offended him. He set it edge up on the top of a bollard, then raised one boot. A full inch of layered oxhide stamped down, studded with iron and moulded by hard usage to intractable curves that echoed the shape of Petro's foot. I knew, through having stumbled over it on many occasions when drunk, that my old tentmate's massive trotter deserved respect.

Petro crushed the ring into a useless twist. Sneering, he handed it back. The state would forego that gold.

`You're enjoying this,' Fusculus tutted, pretending to admonish his chief. Fitted out with a sense of irony, Fusculus must be the sensitive one.