The Temple of Vulcan was down by the Tiber. It was eight days before the Kalends of September, the Volcanalia, the festival of the lame and ugly blacksmith-god. Vulcan had been married off to Venus, the beautiful goddess of love, much against her inclination. She had wasted no time cuckolding him with Mars, the straight-limbed and virile god of war. Discovering the betrayal, the outraged husband had retired to his forge and hammered out a bronze hunting-net, as thin as gossamer but quite unbreakable. Telling his wife that he would be away, he left the trap suspended above the marital bed. Sure enough, at dawn, Vulcan summoned the other gods to witness the illicit lovers naked and entangled. A man of traditional piety, Murena knew the myth well. It was propitious, he thought, for his task. It was the duty of the frumentarii to uncover the secret and the shameful.

The approach to the sanctuary along the embankment smelt of mud and decay and sewage. The yellow-brown waters of the Tiber were low at this time of year. Wide swathes of the sludge of its bed were exposed – the indescribable filth deposited by a city of a million souls.

Suddenly, Murena had the sense that he was being watched. Stopping to tie the laces on his sandles, he surreptitiously studied those in the street. He recognised no one, and there was nothing suspicious. Certainly none of his frumentarii were present. Since taking up his office, Murena had made a point of memorising the faces of all those under his command in Rome. The sense was nothing but a trick of his imagination.

Murena followed the crowd into the old, gloomy temple. Tradition held it to have been founded by Romulus, at what was then a remote place outside the city, so that he could consult the senate in privacy. That seclusion had been the downfall of Romulus – it was here that he had been murdered. After the deed, the senators dismembered his corpse, and each carried away a piece hidden in the folds of his toga. Nothing remained, not even a shred of clothing. They gave out to the plebs that he had been snatched away to the heavens.

The senators remained as treacherous today. Given the opportunity, many of them would still turn on their ruler, hack him into bloody hunks of meat. That was why Murena was in this ancient sanctuary. The target was easy to spot: tall, clad in a snowy toga, a broad purple stripe on his tunic, standing in the space reserved for the Honestiores, the honest men of wealth and position. This senator would be denied the chance to strike down the Emperor.

‘He came here alone.’ The unseen speaker whispered in Murena’s ear. ‘In a litter, he did not stop, talked to no one.’