The boys were indeed nearby, but they were no longer laughing. Something had gone wrong. Nero’s face, naturally ruddy and prone to blemishes, turned a darker shade of red and was twisted by a sudden fury. He hurled his wine cup at Britannicus. The boy dodged and the cup went hurtling past Vespasian’s nose. Startled, the baby Domitian began to wail again.

Britannicus put on an exaggerated expression of shock. “But, Lucius Domitius,” he said, addressing Nero by his birth name rather than his adopted name, “I merely wished you a happy birthday-”

“You will address me by my proper name, brat!” cried Nero. His ringing voice penetrated every corner of the room. The guests fell silent.

Britannicus raised an eyebrow. “But how can I do that, big brother? Earlier today, the augur explained that ‘Nero’ means ‘strong and valiant’ – and you, Lucius Domitius, are weak and cowardly.”

Britannicus’s friend Titus stifled a giggle.

“That’s a lie, you little bastard!” said Nero. “What are you doing here, anyway? Shouldn’t you be eating in the other room with the children?”

Agrippina approached the boys to stop the row. Claudius remained on his couch and seemed hardly to notice what was happening.

Britannicus left the room, followed by a small coterie of freedmen and attendants, the remnants of Messalina’s faction in the imperial household. He carried himself with remarkable poise for a nine-year-old.

Young Titus looked to his father. Vespasian nodded, and the boy left the room with Britannicus. Vespasian shook his head. “That Britannicus – willful and wayward, just like his mother! I should go after the boy. Perhaps I can persuade him to apologize to Nero. I managed to broker a peace between those Celtic tribes up in Britannia, you know. Maybe I can do the same thing here.” He departed along with Domitilla and the infant, who continued to wail.

Paulina returned to her husband’s side. Agrippina joined them. “What am I going to do about that boy?”

“I suppose you mean Britannicus,” said Seneca. “But more to the point, what are we going to do about Nero? He can’t call the emperor’s son a bastard in public. It won’t do.”

Agrippina nodded. “And yet… one does hear rumours about Britannicus.”

“Rumours?” said Paulina.

Agrippina looked sidelong at Titus, as if deciding whether to confide in him, then went on. “Not that the child is a bastard – though we all know what a whore Messalina was. No, there are some who believe that Britannicus is the child of neither Messalina nor Claudius, that their baby was stillborn and Messalina substituted some other child in the crib, eager to present Claudius with an heir. I ask you, does Britannicus look like either of his purported parents?”

“A changeling, you mean?” Seneca snorted. “That’s the sort of thing that happens in old Greek comedies.”

“When it happens in real life, the results are far from comic.” Agrippina turned to Titus. “Senator Pinarius, I make no secret of the fact that I favour astrology and know little about augury. But I wonder, in this case, could augury be of help?”

“I’m not sure what you mean.”

“Might there be a way to interpret the auspices so as to determine the true identity of a particular child? Your skills at divination are so great, and Claudius has such complete confidence in you…” Agrippina peered at him intently.

Unnerved by her scrutiny, Titus glanced at Claudius. His cousin had sunken deep into his couch and was gazing slack-jawed at his wine cup. Then Titus looked at the young Nero, who was over his tantrum and was flirting with one of the younger female guests. Claudius was the past; Nero was the future. Agrippina seemed to be asking for Titus’s help on behalf of the young man who would almost surely be emperor one day, perhaps sooner rather than later. Titus’s first loyalty would always be to his calling as an augur, to strive for the correct interpretation of the will of the gods; but could he not do that and please Agrippina at the same time?

“To determine whether a given individual is a changeling, traditional augury might be of little use,” said Titus carefully, “but there are other forms of divination to which one might draw the attention of the emperor, who is interested in all forms of prognostication. Cousin Claudius recently charged me with compiling a list of every omen and portent reported in Italy, and together we review that list at regular intervals. Only yesterday, in Ostia, a pig was born with the talons of a hawk. Such an occurrence is invariably a message from the gods. Freakish weather, swarms of bees, rumblings in the earth, strange lights in the sky – all require careful interpretation. I have a secretary who closely examines the registry of deaths, looking for any unusual patterns; on a given day, perhaps every man who dies in Roma has the same first name, for instance. You’d be amazed at all the connections you begin to see, when you look for them.”

“Remarkable!” said Agrippina. “But how does one correctly decipher these signs?”

Titus smiled. “The judgement of an augur begins with training but grows with experience. I’ve spent many years studying manifestations of the divine will.” He looked at Nero, noting his large head and prominent brow. “Tell me, has a physiognomist ever examined Britannicus?”

“Not to my knowledge,” said Agrippina.

“Nor to mine,” said Seneca.

“Their branch of science is very specialized. Based on precepts laid down by Aristotle and Pythagoras, they examine the face and the shape of the head for indications of a person’s destiny. Physiognomists talk mostly about the future, but perhaps they can see the past as well. If there is, as you suspect, something… untoward… about the origin of Britannicus, the truth might yet be revealed to the emperor. Yes, I think the first step to discovering the truth might be to summon a physiognomist. I know an Egyptian practitioner – ah, but here comes your son.”

Nero, having sufficiently charmed the young female guest, gathered the folds of his purple-and-gold toga and approached them.

“Brothers!” he said, rolling his eyes, as if to explain his altercation with Britannicus. “You have a brother, don’t you?” he asked Titus. “A twin, Seneca told me.”

“Yes.” Titus sighed. Yet again, Kaeso was being forced into his thoughts.

“Are you identical twins?” asked Nero. The young man’s curiosity appeared to be entirely innocent, but Titus still cringed.

“In appearance, at least when we younger. Otherwise, we’re so different that I should like to think he was… a changeling.” Titus glanced at Agrippina.

“Why do we never see him?” said Nero. “You’re always coming by to see the emperor in his study. Yet we never see your twin.”

“My brother is…” This was not the first time Kaeso’s unsavoury behaviour had caused Titus embarrassment, yet he had never come up with a good way to explain his brother’s complete withdrawal not just from public life but from decent society. How could anyone in the imperial household possibly understand Kaeso’s bizarre beliefs and perverse behaviour? What excuse could Titus make this time for Kaeso? Should he say that his brother was insane? A drunkard? Crippled by illness?

“My brother is…”

Seneca finished the sentence for him: “A Christian.”

Titus turned pale. “How did you know?”

Seneca laughed. “The tutor of the emperor’s son must know a great many things, Senator Pinarius.”

Agrippina frowned. “How can a Roman patrician be a Christian? I thought that was the name for a sect of the Jews.”

“So it is,” said Seneca. “But here in Roma, as in many other cities around the empire, they have recruited others to join their cult. Mostly slaves, one presumes. The Christians actually welcome slaves, and you can imagine why the less reputable sort of slaves find such a cult attractive – Christ-worship is yet another activity they can carry on in secret behind their masters’ backs. But they are not all slaves. I’m told there are a few Roman citizens among the Christians. They teach that this world is a terrible place, dominated by evil men – indeed, they believe that Roma and all it stands for is evil – but they also think this world will soon end, to be replaced by another world, in which their dead god shall come back to life and rule for eternity. A suitable religion, if one can call it that, for disgruntled slaves, but hardly for citizens of the city whose destiny is to maintain order in the world and uphold respect for the gods.”