`Oh Linus, Linus! Oh my darling! Whatever am I going to do?'

`The cohort is ready to support you all it can. The tribune will be writing you a letter -'

`Will I get compensation?'

That was better. It came out smart as a crack of artillery. Petro could deal with that. `I believe there will be a modest award, enough to give you a small pension. Linus was a good officer killed in the state's service -'

`Small!'

`Of course nothing can really replace him.'

`Small, you say! He deserved better. I deserve better for acting as his only solace while he did his cruel job!'

`We all deserved better than to lose Linus.'

We were achieving little, and as soon as it seemed decent we prepared to leave. Rufina then thought of more embarrassment to hurl at us: `Where is he now?'

`Not in Rome yet,' Petronius rapped back swiftly. He had gone very pale. `You don't want to see him. Rufina, don't try!'

`He's my husband! I want to hold him in my arms one final time. I want to know what they did to him -'

Petronius Longus raised his voice so harshly he stopped her. `Remember Linus as he was! What they are bringing to Rome is a six-day-old corpse that has been lying in the open. It's not him, Rufina. It's not your husband; it's not the friend and comrade who served under me.'

`How do I know it is really Linus then? There might have been a mistake.'

I put in weakly: `Petronius Longus will ensure there has been no mistake. Don't upset yourself on that point. He will do what is needed; you can rely on him.'

That was when the widow suddenly crumpled up. With a small gurgle of pathetic grief she fell into Petro's arms and sobbed. She was taller than the girls he liked comforting, older, and her nature was much harder. But he never flinched, and he held her firmly while she wept. I managed to find a neighbour to take over, then we slunk away.

When the carter brought the body to the Ostia Gate, Petro and I were there waiting for it. The customs people had found an undertaker to provide a lidded coffin; Linus came home sealed in state like some general who had died on an intercontinental campaign. But before we passed him on to the funeral arrangers we had brought to the gate with us, my friend Lucius Petronius wrapped a scarf around his face, then insisted that the coffin lid be raised so he could identify his man formally.

As Petronius had warned Rufina, after six days in the sun and salty air this body bore little resemblance to his bright, cheerful, fearless volunteer. The corpse was wearing the sailor's disguise we recognised. It was the right build. The features looked correct. Taken with the identification-tag evidence, we accepted that this was Linus.

Balbinus had taken a stupid risk. He must have been so eager to regain land that he couldn't wait until the Aphrodite left the coastal shallows and found deeper waters where a corpse could be safely pushed overboard and lost. So he brought Linus back to land with him. Someone – the freedmen we had seen leave with him, perhaps – must have helped. Then Balbinus or others had killed Linus, and abandoned his body in a casual manner that was unbelievably arrogant.

I stayed alongside Petronius while he grieved, then I, dealt with transferring the coffin. When the grumbling Ostian carter had removed his vehicle and the coffin had been carried away by officers of the vigiles' funeral club, we two walked back from the Ostia Gate. Once in our nostrils, the smell of putrefaction stayed cloyingly with us. In silence we found our way to the riverbank.

It was now dark. We had the complicated mass of buildings forming the granary area and the Emporium complex on our left, and the Probus Bridge along on the.right, lit by dim lamps. Occasional figures crossed the bridge. We could hear the Tiber shifting, with splashes that could be fish or rats. Across the water, donkey hooves sounded sharply on a road in the Transtiberina. A breeze made us bury our chins deeply in our cloaks, though the air was humid and we were more depressed than cold.

There was no easy way to end this night. Already I felt ominous portents of how it might turn out for me.

`Do you want to go for a drink?'

Petronius did not even answer me.

I should have left him then.

We continued to stare across the river for some time. I tried again. `There's nothing you can do and it's not your fault.'

This time he roused himself a little. `I'm going to the patrol house.'

`You're not ready for that yet.' I knew him better than he knew himself. People never want to hear that happy news.

`I have to tell my men Linus is dead. I want them to hear it from me.'

`Too late,' I said. `Rumour will have rushed straight to them long ago. We've spent more hours on this than you realise. You've lost track of time. On the Aventine this is old news. The whole cohort already know.' I reckoned at least one cohort member knew about this before we did. A fact to which Lucius Petronius still seemed oblivious.

`This is nothing to do with you, Falco. This concerns me and my men.'

I felt the full drag of disaster now. He wanted a quarrel. He needed a bad one. It could have been anyone who caught the eruption, but I was his best friend so I was the rash man who had stayed at hand.

`You're not ready to see them,' I told him again. `There is a situation you have to think about carefully first.'

`I know what needs to be done.'

`I don't believe you do.'

Somewhere in the remote distance we heard the trumpet. After our years in the legions our brains took it in, though we were too absorbed to react. In the Praetorian Camp a watch had changed. I could no longer tell which stretch of the night we were in now. Normally I always knew, even if I awoke from heavy sleep. Now the darkness seemed quite different, the city's noise unlike itself. Events had been moving at an unnatural pace. Emotions had blurred everything. Dawn might be several hours or merely minutes away from us.

At my side I was aware of Petronius giving me more attention. Patiently I explained. I knew we were unlikely to stay friends.

`This job started out as unpleasant, but it's filthy now. You have to accept that fact before you make a move, or you're going to get it wrong, Petro. There are two issues -'

`What issues?'' he burst out angrily.

`Linus' death throws up two stinking problems.' Both seemed self-evident to me. They remained invisible to him.

`Falco, I have a heart full of grief, there are urgent things I need to do, and it's just not clever to hold me back for some piddling irrelevancy.'

`Listen! First, you've got the whole black business of Balbinus Pius. You can leave that one to creep up and depress you slowly if you like, but let's not delude ourselves. Linus must have been killed to stop him reporting that Balbinus came off the Aphrodite pretty well while we were waving him goodbye across the harbour. There are enormous implications: the man is still here. He never left. Balbinus is in Rome. He probably fixed the raid on the Emporium and he hit the Saepta Julia. He killed Nonnius. He killed Alexander. He killed Linus too, of course. Jove only knows what he's planning next.'

Petronius would face it – and deal with it – but not now. He stirred restlessly. I put a hand on his arm. His skin was hot, as if his blood raced in turmoil. His voice was perfectly cold. `What else?'

'Balbinus knew who had to be killed. Somebody betrayed Linus.'

He answered me at once. `It's not possible.' `It happened.'

`Nobody knew.'

`Think how he died! His identity tag was thrust between his teeth. Some swine was making a point that his true role had been exposed. Linus himself had to face up to the fact he had been spotted. He must have died knowing he had been betrayed. You can't refuse to acknowledge it, for his sake, Petro!'