We paid a guide a bunch of coins to tell us how a window was arranged high up, through which sunlight streamed at break of day, falling so the sunbeam seemed to kiss the god on the lips. It was a device created by the inventor, Heron.

‘We know of him,’ I said. Aulus and I once did a job where I had him in disguise as a seller of automaton statues, all deriving from the crazy imagination of Heron of Alexandria. ‘Is the maestro still practising?’

‘He is full of ideas. He will continue until death stops him.’ I muttered under my breath to Helena, ‘I wonder if Heron does magic with door locks? Might be worth exploring.’

‘You boy, Falco! You just want to play with toys.’

We were told that beneath the temple ran deep underground corridors, used in rites connected with the god’s afterlife aspect. We did not investigate. I keep out of ritual tunnels. Down there in the dark, you never know when some angry priest is going to run at you wielding an extremely sharp ritual knife. No good Roman believes in human sacrifice - especially when the sacrifice is him.

Outside, glorious sunlight filled the elegant enclosure over which the god presided. The precinct was surrounded internally by a Greek stoa - a wide colonnade, double height, its columns topped with fancy capitals in the Egyptian style that characterised Ptolemaic building. In a standard Greek market, there would be shops and offices around the stoa, but this was a religious foundation. Nevertheless, the sanctuary was still used by some citizens in the traditional manner as a place of assembly, and being Alexandria it was lively: we were told that this was where the Christian called Mark came ten years ago to set up his new religion and denounce the local gods. Unsurprisingly, it was also where the mob then gathered to put a stop to that. They set on Mark and had him torn to pieces - rather more persuasive than intellectual chastisement, though well in the spirit of hot-headed Greeks whose gods had been insulted by upstarts.

Generally, the stoa had a loftier, more peaceful purpose; there was ample space for the book-loving public to stroll with a scroll from the Library. They could already read a first-rate translation of the Hebrew books treasured in the Jewish religion, which was called the Septuagint because seventy-two Hebrew scholars had been closeted in seventy-two huts on Pharos Island and instructed by one of the Ptolemies to produce a Greek version. Maybe one day browsers would read something by the Christian Mark. In the meantime, people were happily devouring philosophy, trigonometry, hymns, how to build your own siege warfare battering ram, and Homer. Sadly, in the Serapeion Library they could not borrow The Spook Who Spoke, by Phalko of Rome.

Don’t think I was so immodest. Helena asked for me. That way we learned our first hard fact about the Daughter Library: it contained over four hundred thousand works, but they were all classics or bestsellers.

When we met Timosthenes, we congratulated him on the flourishing academy he ran here. He was younger than some of the other professors, slim and olive-skinned; he wore a shorter beard than the old fellows, had a square jaw and neat ears. He told us he had reached his high position after working on the Great Library’s staff. From the look of him, despite his Greek name, he might actually be Egyptian in origin. There was no suggestion that this would make him more sympathetic to our task or likelier to betray confidences, however.

I let Helena talk to him first. Settling your interviewee is a good trick. Lulling him into a sense of false security would only work if he did not realise what was going on, but either way, it allowed me to watch him silently. I knew Helena thought I was subdued because we had not found my play. The truth was, I always enjoyed watching her in action.

‘I know you must be asked the same questions all the time, but tell me about the Daughter Library,’ Helena urged. She looked bright-eyed and curious, yet her cultured senatorial voice made her more than a mere tourist.

Timosthenes willingly explained that his Library at the Serapeion acted as an overflow, carrying duplicate scrolls and offering a service to the general public. They were barred from the Great Library, originally because using it was a royal prerogative then because it was the select preserve of the Museion scholars.

Mention of the scholars led to a distraction, though I put it down as accidental. ‘Someone told me,’ Helena said, ‘there are a hundred accredited scholars. Is that right?’

‘No, no. Closer to thirty - at the most fifty’

‘So my young brother, Camillus Aelianus, was fortunate indeed to be allowed to join them!’

‘Your brother is an influential Roman, connected with the Emperor’s agent. I heard, too, he came with a very good reference from Minas of Karystos. The Board is happy to grant temporary accreditation for someone with such pulling power.’ Timosthenes was wry; not quite rude - yet close.

Helena’s heavy eyebrows had shot up. ‘So was Aelianus approved by the Academic Board?’

Timosthenes smiled at her acuteness. ‘He was admitted by Philetus. Someone put it on the agenda afterwards.’

Helena tossed in, ‘Raised a complaint, I imagine!’

‘You have seen how things work here.’

‘Who called Philetus into question?’ I asked.

Timosthenes clearly regretted mentioning this. ‘I believe it was Nicanor.’ Aulus did study law. So their legal head objected? ‘Nicanor was being difficult on principle.’

Helena said stiffly, ‘My father, the senator Camillus Verus, is set against corruption. He would not want my brother to use unfair influence. My brother himself is unaware that special pressure was applied.’

Timosthenes soothed her. ‘Be calm. The admittance of Camillus Aelianus was discussed and retrospectively agreed by all.’

‘Tell me the truth,’ ordered Helena: ‘Why?’

Helena could be forceful. Timosthenes looked taken aback and fought it with frankness. ‘Because Philetus, our Director, is terrified of whatever the Emperor sent your husband here to do.’

‘He is shit scared of me?’ I interrupted.

‘Philetus is accustomed to run in circles after his own tail.’

That was something. We had induced the man to reveal an opinion.

Timosthenes was a good educator. He was eloquent, content to discuss things with women, revealed no burning grudges. At the same time, he did not tolerate fools gladly - and he obviously put Philetus in that category.

Helena dropped her voice: ‘What makes Philetus so frightened?’

‘That,’ replied Timosthenes in a mild tone, ‘he has not shared with me.’

‘So you do not work in harmony?’

‘We co-operate.’

‘He sees your worth?’

I chuckled. ‘He fears it!’

‘I exercise toleration towards my Director’s defects,’ Timosthenes informed us, po-faced. A short lift of the hand instructed us not to trespass further. Continuing would have been impolite. Saying ‘my’ Director emphasised that this man was bound by professional loyalty.

I decided to be formal. I asked about his hopes for Theon’s post. Timosthenes admitted at once that he would like it. He said he had got on well with Theon, admired his work. But he saw his own chances of being referred for the post by Philetus as so slim that this could not have been a motive to harm Theon. He expected nothing from the man’s death.

‘As Librarian at the Serapeion would this not be a natural career progression? Why does Philetus despise your qualities so much?’

‘It is,’ said Timosthenes heavily, ’because I achieved my post through the administration route, as a member of the Library staff, rather than as an eminent scholar. Although Philetus is himself a priest by background - or perhaps because of that - he is imbued with snobbery about “professors”. He feels it adds to his own glory if the chief of the Great Library is famous for his academic work. Theon was a historian, of some note. I am self-taught and have never published any writings, though my interests are in epic poetry. I am primarily an administrative librarian, and Philetus may feel my approach is at odds with his.’