I reached the Temple alone. Activity and solitude came like a breath of fresh air.

Whatever the crisis, I had to go alone-and on foot. I wore out my boots, but I was keeping my professional integrity intact.

Every time I paid my shoemender, integrity bothered me less.

The Little Temple of Hercules stood in the Aventine Sector, which was where I lived, so I was able to turn up like any local gawker who had spotted the flames on his way home from a bawdy-house and greeted this spectacle as his second treat of the night. It was a pitiful shrine. It had been poked in between a Syrian bakery and a knife-grinder's lockup booth. There were two worn steps where pigeons stopped to gossip, four front pillars, a warped wooden pediment, and a cranky red roof which bore abundant evidence that it was where the pigeons reassembled when they flew up off the steps.

Temples always seem to be burning down. Their building regulations must omit safety buckets and firefighting platforms, as if dedication to the gods brings its own insurance. But evidently the gods get bored guarding altars with unattended perpetual flames.

The fire was well away. There was a lively crowd. I pushed through to the front.

The Aventine vigilantes were leaning in neighbouring porticos while the blaze lit their faces with lurid red. They were a scarred-looking crew, though most had affectionate mothers and one or two could even tell you who their fathers were. Among them my old friend Petronius Longus, a broad, calm, square-browed officer with a baton through his belt, stood thoughtfully cradling his chin. He looked like a man you could drag into a corner for a natter about women, life, and where to buy a hock of Spanish ham. He was captain of the watch, but we never let that interfere with being friends.

I squeezed in alongside. The heat felt strong enough to melt the marrow in our bones. We scanned the crowd in case there was a mad-eyed arsonist still lurking at the scene.

'Didius Falco,' Petronius murmured, 'always first back into barracks, hogging the fire!' We had both done army service in the bitter north: five years in the Second Augustan Legion in Britain. We had spent half our time on the frontier, and the rest on forced marches or camped out in the field. When we came home we had both sworn we would never feel warm again. Petronius married; he decided it helped. Various eager young ladies had tried to assist me the same way, but I had fended them off.

'Been visiting your girlfriend?'

'Which?' I grinned. I knew which. For at least the past fortnight there had only been one. I set aside my vivid recollection of offending her this evening. 'What highly avoidable accident happened here, Petro?'

'Usual fiasco. Temple acolytes off playing dice in a bar down the street; an incense burner left smouldering…'

'Casualties?'

'Doubt it; the doors are locked-' Petronius Longus glanced at me, saw from my face there was a reason for the question, then turned back to the temple with a heavy groan.

We were helpless. Even if his men burst those studded double doors with a battering ram, the interior would explode into a fireball. Flames were already flickering high on the roof. Black smoke with a worrying smell was gusting halfway to the river. Out here in the alley the heat was making our faces shine like glass. No one could survive inside.

The doors were still standing, and still locked, when the roof-timbers caved in.

Someone finally rooted out the fire brigade from a chophouse to douse the shell of the building with buckets. They had to find a working fountain first, and it was the usual ham-fisted effort when they did. Petronius had dispersed most of the crowd, though a few characters with fierce wives waiting at home clung on here for the peace. We hooked grappling irons onto one of the doors and dragged its scorched timbers outwards with an ear-splitting screech; a solidified torso, presumably human, lay huddled just inside. A professional priest who had just arrived told us the molten amulet stuck to the breastbone looked not unlike one Curtius Longinus, the conspirator recalled by Vespasian, always wore.

Longinus had been his house guest. The priest had dined with the man that evening; he turned away looking sick.

Petronius Longus yanked a leather curtain over the charred nugget of flesh. I let him start the questioning while I went on looking round. 'Do you normally lock the doors at night?' he challenged, coughing in the smoke.

'Why should we lock up?' The priest of Hercules had a healthy black beard; he was probably ten years older than us but looked hard as the Citadel Wall. You would only play handball with this stalwart cove if he picked you to play in his own team. 'We're not the Temple of Jupiter, crammed with captured treasure, or the Temple of Saturn Treasury. Some shrines have to be shut up at dusk to stop vagrants creeping in, but, watch captain, not ours!'

I could see why. Apart from the fact gruff old Hercules Gaditanus probably liked vagrants, there was nowhere to squat in comfort and nothing to steal. It was just a brick-built closet no bigger than a storeroom on a farm.

The terracotta statue of the god which had been laid low by a ton of falling roof tiles had a half-finished air that went with the rough-and-ready place. Even his priest had the famished look of a man who worked in a poor district, dealing all day with brain-battered boxers. Beneath the beard, his oriental face was handsome; he had great sad eyes, as if he knew his god was popular but not taken seriously.

'Who was in charge?' Petronius continued wearily, still upset by the death. 'Did you know this man was here?'

'I was in charge,' stated the priest. 'Curtius Longinus had an interview tomorrow with the Emperor. He was praying in the Temple to compose himself-'

'Interview? What about?'

'Ask the Emperor!' snorted the priest.

'Who keeps the Temple key?' I interrupted, inspecting what was left of the sanctuary.

'We leave it on a wall hook just inside.'

'Not any more!' Petronius corrected angrily.

The hook was there: empty. I stepped over to see.

The priest gazed helplessly at the smoking shards of Hercules' stricken house. Sparks on the inner walls still raced up cracks in the lining cement. He did not want to distress himself surveying the damage while Petro and I were watching him.

'I must write to his brother…'

'Don't do that!' I ordered him coldly. 'The Emperor will inform Curtius Gordianus himself.'

The priest began moving off so I prepared to follow. I nodded to Petro, who jerked his head back, annoyed at me for rushing away. I thumped his arm, then clambered out after the black-bearded fellow.

Emerging, we passed an excitable figure who worked for Anacrites; he was so busy making his own presence felt he missed us going by. When I glanced behind he was harassing Petro. Petronius Longus planted his large feet apart and just listened with the faraway look of a tired man who badly needs a drink, planning in advance whether to have half an amphora of his usual crimson rot-gut and a terrible night's sleep, or to broach the delightful Setinum he has been nursing along at the back of a shelf… The spy was getting nowhere. Peaceful insolence is a speciality of the Aventine watch.

As the priest set off homewards I skipped along too.

'Did Curtius Longinus arrive back in Rome tonight?' He nodded in silence. Shock had settled on him now; he did not want to talk. His mind was preoccupied but his legs walked automatically with long muscular strides; it took energy to keep up without losing my dignity. 'So he had no chance to meet anyone?' He shook his head.