We raised a quiet toast, then set our beakers down again as Petronius prepared to speak.

‘Is this about our Modestus case?’ Having been to the marshes with us, Quintus was over-keen and jumped in. I laid a finger to my lips. Good-natured, Quintus shrugged an apology.

Petro began slowly. ‘Marcus Rubella, my tribune, introduced Silvius to me, but officially, Rubella never met Silvius—and nor have I. Officially we surrendered the case into the safe hands of the honest Praetorians, together with their intellectual comrade, Anacrites the spy. There’s a poor interface with his organisation. We all let Anacrites play by himself.’

Aulus asked, keeping his voice level, ‘Who are “we all”? The vigiles, the Praetorians, and whoever Silvius’ people are?’

Petro gave a satirical growl. ‘Here is how co-operation works, boys.’ He branched into a lecture I had heard him give before: ‘The Praetorian Guard provide the Emperor’s security—hence the link with the intelligence outfit. Titus Caesar commands them, to keep them under control—though who will control Titus? They spend a lot of time nowadays arresting people whose faces Titus does not like. Upset Anacrites, and that could be us. The Urban Prefect is Rome’s city manager. Duties include investigating major crime—note that. Then come the vigiles. Duties: sniffing out fires, apprehending street thieves, rounding up runaway slaves. When we catch minor criminals, we give them on-the-spot chastisement—otherwise we parcel them up for the Urban Prefect, who charges them formally. So another point to note, Aelianus: we have good lines of communication with the Urbans. Very good.’

I leaned on one elbow and pointed one forefinger at Silvius. Silvius nodded. He belonged to the Urban Cohorts.

The Camilli watched this interchange. Justinus asked pointedly, ‘The Guards and the Urbans live in the same camp. Are they not natural allies?’

‘You might think so,’ admitted Silvius. ‘Though not for long. Not once your keen eyes observed how the Praetorians behave like gods, looking down on the Urbans as their poor relations—while also thinking that the vigiles are puny ex-slaves, commanded by has-been officers.’ Petronius spat out an olive stone. ‘Pity the pathetic Urban who has bought the myth that it is easy to pass from one section to the other, merely on talent and merit,’ Silvius continued in complaint. I wondered if that was what he had tried to do, and failed. ‘No vigiles officer, I suspect, would even waste his time thinking it could happen.’ Ah. Tell that to Marcus Rubella, whose dream was to rise on snowy wings to wear the Praetorian uniform.

‘So you work in Rome,’ Aulus pressed Silvius.

‘Personally, no.’

We all raised our eyebrows—except for Petronius who calmly supped his drink and waited for Silvius to explain.