Inches taller than me to start with, he even seemed to have grown. His quiet manner tended to disguise how powerfully built he was. Slow of step and wry of speech, he could lean on wrongdoers before they even saw him coming, but once Petro applied weight, resistance caved in fast. He ran the watch enquiry team without seeming to exert himself, although as his best friend I happened to know that in private he worried deeply about standards. He achieved the highest. His was a lean, competent squad which gave the public what they paid for, and kept the villains on the hop.

He had a calm grip on his domestic life as well. A good Roman: honorific father of three children. He had a small; scathing wife who knew how to make her presence felt, and a much-loved trio of lively little girls. At home he fielded Arria Silvia's sparky temper pretty easily. The children adored him. Even the wife modulated her complaints, knowing she had one piece of fortune that was missing from most marriages: Petro was there because he wanted to be. Both, as a family man and as a public officer, he looked easygoing but was utterly reliable.

`Balbinus Pius…' he said softly, savouring his triumph.

`Ludicrous name,' I commented. `Balbinus the Dutiful! As far as I know his only duty is serving himself. Isn't he the mouldy cheese who owns that filthy brothel they call Plato's Academy? And the thieves' kitchens down on the waterside at the back of the Temple of Portunus?'

`Don't speak to me about Plato's. I get a pain in the bladder just thinking about the place. Jupiter knows whose name is scratched on the crumbling title deeds, but you're right, it was Balbinus who had it sewn up. He took a percentage of every transaction in bed, plus whatever the house made on robbing purses or selling "abandoned" boots and belts. Then, as well as his entertainment interests, he had a nice goldsmith's workshop where stolen goblets could be melted down in minutes; several sweatshops that specialised in putting new braid on tunics that "fell off" washing lines; numerous tat stalls in the markets, constantly shifting just when I placed a man in the portico watching them; and a couple of counterfeiting factories. If it stank, he owned it,' confirmed Petro. `Past tense, though, Falco. One of the bleak facts he has to face today is that a capital conviction means losing all his property.'

`I'm sobbing into my napkin.'

`Don't upset yourself too much – I'm still not certain we'll net his whole empire. Some of it must be in hidden hoards.' `I bet! Was he expecting to be put away?'

`He wasn't even expecting to be put on trial! This has taken me months of planning, Falco. There was only ever going to be one crack at him, or he'd be screaming "persecution of a citizen!" and I'd be out of a job. But he didn't believe I'd ever find anybody prepared to prosecute.'

`So, Lucius Petronius, how did you arrange. it?'

`Marcus Didius, there was only one way possible. I found somebody even greedier, and even more of a bastard, than him!'

SMILING, PETRO PASSED one big hand over his brown hair. He seemed too have been having it styled more snappily. (Well, it was shorter; that was his barber's creative limit.) His other great paw lay lightly at his waist, where the staff of his office was stuck behind a wide, creased leather belt that I remembered him buying from a shifty Celt in Londinium. Otherwise, apart from the flash haircut, he did not trouble to priss himself up like a man of fashion. On duty it was better to be protected by a leather jerkin that might deflect a knife blade and a thick wool cloak which would shrug off the mud if he hurled himself to the pavement when tackling a runaway. His boots had come up hard on quite a few doorframes too by the looks of them.

`So who was the high-principled, public-minded citizen who squealed about Balbinus?' I asked.

`A donkey's turd called Nonnius.'

`Not Nonnius Albius? I thought he was a racketeer himself?' `He had been. He actually worked with Balbinus, was his chief rent collector. That was what appealed to me.' `Of course! You needed an insider.'

`No one else could have done it. Nonnius was ideal.'

`But he was a Balbinus boy. How did you sew him up?'

`A sad story.' Petro grinned. `He's dying. His doctor had just put the frighteners on. Pool old Nonnius is suffering from terminal rot.'

`Something nasty that people don't talk about?'

`Same as his profession!' Petro snarled. Then he told me the story: `Back in the spring, I just happened to learn that Nonnius had been given notice to quit by his pet medicine man 'Happened?' This seemed a nice coincidence.

Petro was in full flow and not to be sidetracked by my scepticism. `Nonnius gets informed by some pet Aesculapius that he's finished, but the doctor says he'll last longer if he takes care of himself – no worries, lots of pampering -'

`Expensive!' I was beginning to see Petro's reasoning.

`A life of luxury prescribed! So I get to him when he's just reeling from the bad news, I lend a sympathetic ear, then I put it to him he's spent his life running around for Balbinus while that rat lay on a reading couch counting his winnings – and for what? Now seems the time for a spot of levelling… Since Nonnius has to give up the low life, he soon settles on snatching at the high life to compensate. This appeals to the bastard: taking a litter through the Forum, giving orders to slaves through the window and greeting fawning admirers who are hoping for free gifts. Even more than that, suddenly he loves the idea of robbing Balbinus.'

I laughed shortly. `The loyalty of thieves! So he was prepared to testify?'

'In return for the traditional reward.'

`You did a deal?'

`All legal. He appeared before Marponius and twittered like a happy song finch. In return, as a successful prosecutor he can seize a proportion of Balbinus' traceable assets. The only disincentive is that he has to help us trace them. But it's well worth his while to hire accountants. Having been on the money-collecting side himself he knows the occasional fellow with a dodgy abacus, imaginative enough to guess where the loot may be hidden.'

`I love it!' I was laughing. We both grabbed more wine, which now tasted almost palatable. `But Petro, you must have needed to take great care framing the actual charge against Balbinus. What did you throw at him?'

`Murder. The only count that would have worked.'

`Of course. It had to be a capital offence.'

`Right. Anything less and he would only end up with a fine and however large, a fine wouldn't choke him. He could shed thousands and hardly feel a tickle.'

I didn't say it, but putting Balbinus in court on any charge that left him free in Rome afterwards would have placed Petro himself in a very dangerous position. There was no point dwelling on this feature. He knew all right.

`So who had been topped – and how did you nail Balbinus for the murder?' I didn't suppose he had actually stuck a dagger in someone personally. `Getting blood spots on his own tunic was never his style.'

`Happy accident,' said Petro. `It happened at Plato's Academy.' The brothel we had already mentioned… `They specialise in fleecing foreign visitors. Some poor Lycian had been set up to lose his travelling pouch in the floor-creeping gag. While the girl was giving him the push-and-shove that he'd paid for, he made the mistake of noticing a rustle in the straw. Up he jumps, and discovers the whore's accomplice just reaching for his money. Instead of making a discreet complaint to the madam, then leaving the brothel with an apology and a wiser attitude, this fool puts up his fists and makes a fight of it. The snatcher was so surprised at the Lycian's unsporting behaviour that he knifed him on the spot.'