Mevania is a beauty spot a hundred miles north of Rome. Agrippina chose it as the gathering place for her fellow conspirators. A lovely setting to plot mayhem and bloodshed. The villa was cool with well-watered lawns, enclosed gardens, peristyle-shaded walks. It was some distance from the road, and Agrippina had it carefully guarded with all approaches watched. Despite my protests, the conspirators were invited and arrived one by one. Progeones, of course. Lepidus with his long head and shock of black dyed hair: a born conspirator with his twisted smile and bitter, cynical eyes. His weak, furrowed face mirrored unresolved grievances and spite. Then came a brilliant orator, a small, dapper man with flickering eyes and a surprisingly deep voice, dumpy legs and a chest like a barrel. I had heard him speak in the courts: he was brilliant. Agrippina planned to use him to turn the Senate.

Uncle Claudius should have arrived but failed to do so. Instead his representative Seneca made his first appearance in my life. Seneca, a Spaniard, looked every inch the Roman patrician and philosopher. He was of medium height and well built, with a strong, broad face, aquiline nose, and snow-white hair carefully combed forward. Seneca looked like a pompous Platonist except for the shrewd cast to his mouth and those deep-set eyes, which viewed the world with cynical amusement. Seneca had no doubts about the rightness of Agrippina’s cause. He reminded us all of Arrentius’s last words, only this time he recast them: ‘If life with Tiberius was bad enough, life with Caligula has been pure hell!’

Admittedly I was most uneasy. If you are going to form a conspiracy you must trust everyone involved. I knew little about these plotters. Time and again I broached the matter with Agrippina, but she acted as if she was possessed. She wasn’t so concerned about Caligula, more with the child that Caesonia was expecting. One evening, at the end of December, all was ready. The conspirators, or their leaders, gathered in Agrippina’s bedroom, a place of dark damascene cloths, jewelled cups, gold and silver statuettes, expensive furniture of oak, maple and terebinth, ivory-footed couches, stools and chairs made of tortoiseshell. Her large bed dominated the room. It was carved out of rare wood which reflected in its undulating grain a thousand different shades of colour, like that of the great peacock feathers adorning the wall above it.

Her son was not there. She had left him with trusted nurses in her house on the Via Sacre. We discussed how and when Caligula should die. Agrippina finally made a decision.

‘Rome would be too dangerous,’ she reasoned. ‘And when Caligula reaches Germany, he’ll be too well protected. I’ve invited him to visit me. We must do it here!’

‘By poison?’ Seneca asked.

Agrippina disagreed. She opened a leather bag and emptied three silver-embossed daggers out on the table.

‘It will be done publicly enough,’ she continued. ‘And I will take responsibility.’

Agrippina had assumed the role of the democrat eager to save the republic from a tyrant. She laughed as if aware of her theatricality and looked at us from under dark, arched eyebrows.

‘Who will strike the blow?’

‘I will not,’ I replied, getting to my feet. ‘Nor will I take the oath.’

‘Why not?’ Agrippina asked.

I left and walked into the coolness of the garden. The murmur of voices rose from behind me, the sound of a door being firmly closed. I was in a sulk. I hoped Agrippina would have followed me out but I was left to kick my heels.

At least an hour passed before she joined me. ‘The others claim you can’t be trusted,’ she said, sitting down beside me.

‘Well, say that I don’t trust them.’

‘What do you mean?’

Agrippina slipped her arm through mine and pulled herself closer. I smelt her delicious perfume, or was it a soap she used after bathing? Light, fragrant but still cloying to the nostrils.

‘Oh, I trust them, I suppose,’ I confessed. ‘But I don’t trust Caligula. He let you come here. He may be mad as the moon but he must suspect: someone in this villa is his spy.’

Agrippina refused to agree. Two days later Caligula arrived in a gorgeously decorated chariot pulled by four beautiful bays, their manes starred with special gems, breast-plates covered in sacred amulets. He had changed his role, now he saw himself as Charioteer of the Gods. Caligula himself stood upright like a victor about to prepare to receive the palm, helmet on his head, whip in hand, leggings of gold and red covering calf and thigh.

Of course, we all had to watch him drive up and down the gardens of the villa, creating chaos every time he turned. Lawns and flowerbeds disappeared, small, delicate walls were sent crashing under the spinning wheels. A group of Praetorians accompanied him and, of course, Castor and Pollux, his two German shadows. Caligula was frenetic with excitement. After a while he tired of the game, climbed down from the chariot, undressed in front of us and charged into the villa demanding a bath, his tunic and toga.

Agrippina entertained him late that evening. She tried to hide her own unease behind the pretence of a lavish banquet: hens made of wood containing eggs were brought in on platters, dormice rolled in honey and poppy seeds; hot sausages mixed with grilled damsons and seeds of pomegranate; wild pig, boiled carp and large jars of heavy Falernian. Caligula refused to eat unless Castor or Pollux tasted the dish first, whether it was a huge lobster, garnished with asparagus, or lampreys from the straits of Sicily. He even poked his dagger at the truffles and delicious mushrooms. After a while he threw the dagger onto the table and gazed around.

‘I’m off to Germany,’ he declared, and paused, head cocked to one side. At first I thought he was in one of his mad trances till I heard the clink of metal and the tramp of feet. I sprang to my feet, looked through a window and glimpsed pinpricks of torchlight: fresh troops were arriving. More torches appeared, and from outside came the sound of running feet. I heard a scream from the kitchen.

‘I thought I’d supply the entertainment.’

Caligula swung his feet off the couch and stared evilly at his sister. Agrippina kept her poise. I felt my arm grasped. Castor had crept, as quiet as a cat, up beside me. He grunted and gestured with his hand that I re-take my seat.

‘I’m off to Germany,’ Caligula repeated, ‘but, before I go, I must deal with traitors. Right, Progeones, tell your story!’

Our horrid little gargoyle sprang to his feet. Like an actor who has scrupulously learnt his lines, he confessed everything. Agrippina sat white-faced. Lepidus tried to rise but one of the German bodyguard thrust him down. In the darkness behind the Emperor I heard a door open and the hiss of drawn swords as more of his bodyguard arrived. Once Progeones had finished, Caligula clapped, at first softly then louder and louder.

‘That’s the first part of the entertainment!’ Seneca began to cough, spluttering over something he had eaten.

‘Don’t spoil the entertainment!’ Caligula shouted. ‘Take the old fool away!’

Seneca was hustled out.

‘Don’t kill him!’ Caligula shouted over his shoulder. ‘I want to watch our philosopher die! See if he accepts death with the same equanimity as he faced life. Lepidus.’ Caligula looked back at his guests. ‘Lepidus,’ he cooed.

The senator was seized and brought before the Emperor. Caligula swung his foot and cleared the table with his boot, sending dishes, cups and platters flying. Lepidus was forced to sit on the edge, with Castor and Pollux on either side. Caligula picked up a fork that had been used for the sucking pig. With one swift jab, he expertly dug out Lepidus’s right eye. The man screamed and tried to rise but the guards held him fast.