I rushed after Diogenes and his two mysterious trackers. When I caught up, he was moving along briskly, but as if he was disguising the fact he was trying to get away. The men in desert clothes were following about five strides behind, one on each side of the road. I kept them all under observation until Diogenes hit the agora.

The market lay close to the heptastadion, the Pharos causeway. It was a huge square enclosure, open to the skies, as large as you would expect in a city devoted to international commerce, which had been established by a Greek. They love their markets. Since Alexandria was a city that hardly slept, most of the stallholders were still working. A rich odour of street food hung like a smoky cloud above the area. Shouts resounded. Wheels rattled. Footloose musicians, barefoot and threadbare, pattered on hand drums and tooted peculiar pipes. It was well-lit and vibrant, somewhere that a trader who knew his way around the city, might well feel he could lose a couple of wild men in dark cloaks who were harassing him.

At first it only looked like a man moving fast between the stalls, with others perhaps trying to catch his attention so they could all go for a drink. I was bemused but game. Where they went, I followed.

Soon it became more sinister. Diogenes began to show panic. Dropping all pretence that he was just walking somewhere and had not noticed any pursuit, he knocked into the corners of a couple of the stalls; he clattered through a pile of metal cauldrons; he kicked aside giant sponges; he annoyed people; he was chased by dogs. I fixed on him. One or other of the two men in cloaks was visible from time to time. It became apparent they were stalking Diogenes as if it was a game. They could have caught him at any time, but they were teasing - they let him think he had lost them, then swooped bat-like out of nowhere, so just as his heart started to settle, he had to be off again.

I suspected Diogenes knew them. He certainly knew what they wanted. The way he had taken off, abandoning the precious scrolls, said it all. A man who had struck me as afraid of nothing was now extremely worried.

The pursuers worked well in tandem. They seemed close-bonded. Perhaps they were Rhakotis residents, or perhaps they had fished and hunted wildfowl together in the great reed-beds of Lake Mareotis. Perhaps they came from those houseboats, where the driver had told Helena and me murderous gangs lived, unchecked by the authorities.

People began to notice the chase. The few women present gathered their children and hurried away, as if they feared trouble. Men stood and watched, though guardedly. Roaming dogs were harshly ordered back. One or two then stood by their owners’ stalls, barking defiantly. A man caught my arm and pulled me to a halt; he shook his head, wagging a finger to warn me not to involve myself. I broke free and heard him mutter a baleful comment as I went on.

I saw a flash of red: soldiers. They were making their way towards Diogenes, though more curious than purposeful. A man with a great basket of apples barged into their way, perhaps deliberately, and sent fruit bowling crazily in all directions; the soldiers just stood there while he let out a cascade of complaint. If Diogenes spotted the military, he made no attempt to appeal for help. He was near enough, but instead moved on. One of his pursuers appeared, but Diogenes grabbed the awning ropes of a tunic stall, heaving over the whole edifice to block the man; entangled in garments, he let Diogenes flee. I jumped over a display of ceramic bowls, tripped on wet vegetable leaves, dodged around the end of a long row of ornament stalls, forcing a path through the crowds as best I could. When I lost sight of Diogenes, I kept going forwards and I had him in clear sight when he made what seemed to me a big mistake: ducking his head and running at a lope, he left the market on the seaward side. He set off down the enormous causeway, the heptastadion. I was so close at that point, I even yelled his name. He looked back, his face anxious, then turned away and speeded up.

The heptastadion looked long enough in daylight; it must be almost half the distance of the city north to south. I was tired, and this chase was not of my making. I decided to return to the agora and alert the soldiers. Let them catch Diogenes. Legionaries could put a roadblock on the causeway and flush out the fugitive at their leisure.

What stopped me was a dark huddle of men outside the agora gate. The rough inhabitants of Rhakotis had answered some call; they were drifting in and suddenly I saw that the gathering was being orchestrated by the two cloaked figures who had chased Diogenes. They were gesturing in his direction as he headed out across the long mole. Poor as they were, I knew the descendants of the scroll pirates would be armed - and vicious. Uncle Fulvius said they were considered very dangerous. When the first few began to make a move, I turned back and on to the causeway.

With no real plan - was I warning him, helping him or hunting him down myself? - I too began running down the heptastadion after Diogenes.

It was a serious hike. The mole was a man-made granite structure, easily as long as its name said: seven stades. At least it was good underfoot. On it ran a decent road, well built to take the fuel convoys for the Pharos and the many daily tourists. Now, in the dark, it seemed almost deserted. Diogenes took it steadily. So did I. So too did the desperadoes behind us. To anybody watching from the shore, or from the packed vessels in the huge Western and Eastern Harbours, we must have looked strung out like a group of athletes in a Pan-Athenian race stadium. We adopted that steady, long-distance pace marathon runners have, saving ourselves at this stage, nobody yet making his move to overtake.

It was a wonderful night. A cool breeze was in our faces, the sky now dark overhead but fizzing with a multitude of tiny stars. Thousands of ships were moored to right and left of us, dark hulks whose rigging made interminable noises, their bumboats splashing and bumping against them in the gently lapping harbour waters. Occasional cries sounded from the dark shore or indignant seabirds let out squawks as their privacy was disturbed. It was too late for casual strollers. If there were lovers or fishermen out there in the gloom, they lay low and kept quiet. On the far side of the Eastern Harbour, I made out buildings faintly lit - the palaces, administrative quarters and other monuments where no one made economies in lamp oil. Any junkets, recitals or concerts would now have ended. Only night-watchmen would be walking the silent marble corridors, though perhaps in some lonely room, by light from a fine wax candelabrum, the Prefect wrote his interminable reports on nothing, to let the Emperor believe he did some work.

I could have been a clerk. I could have allocated sacks and scrawled on dockets. I really could have been a poet. I would have been a poor man, with starving children, but danger would never have approached me . . .

I stopped thinking.

Down seven stadiums’ length we ran, until breathing hurt my chest and my legs felt as heavy as waterlogged wood. I reached Pharos Island. Everywhere was dark. I could no longer see Diogenes. The road forked. Somewhere to the left was a Temple of Poseidon, the great sea god of Greece and Rome, guarding the Western Harbour’s entrance. To the right lay another temple, that of Isis Pharea, the Egyptian protectress of ships. Beyond her, was stationed the Lighthouse, forming the mighty endstop. I went right. The Lighthouse, which must be manned at night, seemed the less lonely destination.

Pharos Island was a curved rocky outcrop, far enough beyond the city to feel like a wild citadel out in the thundering seas that famously beat upon the long, low shores of Egypt. Here, Homer said, Menelaus and Helen were beached during their journey home after the fall of Troy; at that time they found only a lonely fishing village on the island, with seals basking on the rocks. Apart from the Lighthouse, the place seemed uninhabited now, though I could not bank on it.