‘Have you ever really studied Caligula, Parmenon? Does he look like me? Or Drusilla? Or Julia? My mother bore him but that does not mean he is Germanicus’s son.’

‘You could lose your head for that,’ I whispered back. ‘And your son would disappear into a pit.’

I placed my hand on her shoulder — when we were in private she allowed such liberties.

‘Are you going to kiss me, Parmenon? Make love to me?’ she teased.

I never knew what would have happened if I’d tried. Perhaps it was her coldness which always stopped me. Agrippina regarded sex as a gladiator did a sword or shield, a weapon to be used.

‘Or are you going to force me?’ she grinned. ‘Like Metellus did?’

‘I am going to warn you,’ I advised. ‘Progeones is in this house: he would betray you at any time.’

‘Nonsense! He’s mine and always will be.’

‘If he heard what you’ve just said,’ I went on, ‘he would sell the information to any of your enemies who would not hesitate to use it. The slightest hint that the Emperor is a bastard would bring about the cruellest punishments.’

Agrippina swallowed hard and broke free from my grip. She went over into a corner, crossing her arms like a young girl being scolded by her father.

‘I’ve heard rumours,’ she said. ‘Even my mother once hinted at it. That could be why Caligula and poor Drusilla became lovers, since the blood-tie was not so strong.’ She sighed. ‘Now she’s gone. I thought we could keep Caligula distracted with one woman or another but Caesonia is different: she’s as fertile as a brood mare. Caligula is still a young man. In five or ten years Rome could have a nursery full of “Little Boots”.’ She rubbed her hands together. ‘I am sure Caesonia will become pregnant, but Parmenon, we cannot allow her, her husband or any offspring to live.’

‘If Caligula dies?’ I asked. ‘Do you think the Senate will accept your baby son as Emperor?’

Agrippina shook her head. ‘He’s of Germanicus’s line but he’s still too young. No, Uncle Claudius will do nicely for the moment.’

‘Him!’ I exclaimed. ‘That doddering idiot!’

‘He has the imperial blood, Parmenon. If the Senate accepted a madman like Caligula, they’d take a baboon from Africa.’

That was Agrippina’s one great weakness. She would not listen and, once she decided to act, did so impetuously.

A stream of visitors began to call secretly at her house. Agrippina was sifting which ones would listen to her, who was sympathetic? Whom could she trust? Slowly her plan began to develop: Caligula’s speedy assassination in Rome would be followed by letters to the legions on the frontiers. I watched helplessly. She would not be advised or warned. There were three main plotters: Agrippina, her sister Julia and Drusilla’s former husband Lepidus. Julia was involved because she was terrified of her brother. Lepidus, after Drusilla’s death, had fallen from favour. Progeones and I became unwilling bystanders and spectators. I was used as a messenger. Often at night I’d slip along dark streets carrying cryptic messages to various houses. I would deliver these faithfully word for word, before taking an answer back to Agrippina.

It was a dangerous time! Caligula’s madness worsened by the week. He turned up at the Senate and terrified everyone by saying how marvellous Tiberius had been, how wrong they all were to criticise him. This speech marked a renewed persecution. The prisons filled. Caligula liked to visit the torture chambers, eating and drinking whilst his victims experienced a slow, agonising death. Caligula would advise the executioners to go about their work slowly so that the victim would know he was dying. Agrippina, despite her plotting, still tried to restrain him. She sent begging letters but Caligula’s only reply was:

‘Let the people hate me as long as they fear me!’

Justice was sharp and cruel. Parents had to attend the execution of their own children. One father was forced to watch his son die and then invited to dinner immediately afterwards. Caligula joked and jested throughout the meal. The owner of a school of gladiators who had displeased him was beaten to death with chains. Caligula would only allow the corpse to be removed when the stench from the putrefied body became too great. Writers were burnt alive in the arena. A Roman knight was tossed to the beasts in the amphitheatre. He ran across the sand and begged Caligula for a pardon, claiming his innocence. Furious, Caligula ordered that his tongue should be removed before he was thrown back to the waiting lions. Other more hideous punishments were perpetrated. At one infamous banquet he had the hacked limbs and bowels of a senatorial victim stacked in a steamy heap on a table so all the guests could see. Caligula broke the brooding silence with a mad fit of laughter.

‘Don’t you realise?’ he shrieked. ‘I could have all your heads with one cut!’

No one was spared. He had Caesonia, his new wife, paraded naked before guests, accusing them of treason if they looked, and demanding whether his wife disgusted them if they turned away.

By the time the summer heat reached Rome, Caligula was tired of the city. He’d grown particularly concerned by a prophecy given to Tiberius that Caligula had no more chance of becoming Emperor than of riding over the Gulf of Naples on horseback. Caligula was determined to prove this wrong. He marched his troops down to the bay and ordered his engineers to build a bridge more than three miles long from Puteoli to Baiae. Merchant ships were anchored together in a double line and a road, modelled on the Appian Way, built across them. So many ships were commandeered that the corn imports from Egypt suffered. Caligula didn’t care. He arranged for wayside taverns to be built on this makeshift road, together with resting places, even running water was supplied.

Caligula proudly proclaimed that even the God Neptune was frightened of him. The bridge was finished and Caligula had decided it was time to prove the prophecy wrong.

‘You are coming with me, sister!’ he yelled at the banquet held the night before. ‘And you, Parmenon. You’re my lucky mascot, Parmenon. Do you know that?’ His cadaverous face broke into a wolfish grin. ‘I have met him, you know,’ he whispered to me, filling my cup to the brim so the wine splashed out over my hands.

‘Who, Excellency?’ I replied.

‘Tiberius,’ he whispered. ‘He comes to my bedchamber, drenched in blood. What a hideous sight!’

‘Your Excellency, he didn’t die of wounds.’

Caligula grinned, winked and tapped the side of his nose. ‘You didn’t see what I did to his corpse afterwards,’ he replied. ‘I did enjoy myself.’

And then he turned away to bestow slobbering kisses on Caesonia. Agrippina, on the couch before me, watched this red-haired, florid-faced woman intently. My mistress reminded me of a cobra about to strike. Once we were away from the banquet she turned to me.

‘The bitch is pregnant!’ she murmured. ‘It’s time we acted!’

The following day Agrippina and I joined Caligula in a splendid chariot. The Emperor wore the breast-plate of Alexander the Great, ransacked from the Conqueror’s tomb in Alexandria. He also insisted on wearing full armour, a purple cloak trimmed with gold and adorned with jewels from India, as well as a crown of oak leaves. He then made sacrifice to Neptune and rode across his makeshift road. Backwards and forwards we went, both that day and the next, until I thought I would drop. Caligula rewarded his soldiers and invited all the onlookers onto the bridge.

The celebrations became frenetic. Many became so drunk and incapable, they fell off: corpses were washed up on the sands for weeks afterwards. Agrippina was furious, not so much with her brother’s madness, more that he might have a possible heir.

When we returned to Rome, she made a decision.

‘Caligula is to go to Germany. We must make sure he never arrives there.’ She took a bracelet off her wrist. ‘Give that to Lepidus. Tell him the die is cast!’