I ascended the stairway to the upper storey. I must have turned down the wrong hallway and somehow lost my way. Tiny lamps lit the passages at intervals, and windows let in shafts of moonlight, but still I found myself confused. I tried to determine the direction of the bay by listening, and instead found myself hearing the faint gurgling of hot water through Orata's much-esteemed pipes where they were invisibly laid beneath the floor and along the walls. I passed a closed door and thought I heard faint laughter within – the deep voice of Marcus Mummius, I was almost certain, and another, softer voice replying. I walked on and came to an open doorway from which came a steady, raucous snoring. I took a step inside, squinting in the darkness, and saw what appeared to be the bulbous profile of Sergius Orata reclining on a wide couch with a gauzy canopy. I returned to the hall and pressed on until I came to the semicircular room where Gelina had greeted us earlier.

'Gordianus the Finder' you call yourself, I thought with disgust, thanking the gods that no one was there to laugh at me. I had come to the northern end of the house, having turned in exactly the wrong direction after I ascended the stairway in the atrium. I was about to turn back, when I decided to step onto the terrace for a breath of air to clear my head.

Beneath a waxing moon, the bay was a vast expanse of silver scalloped with tiny black waves and circled by black mountains pierced here and there with a point of yellow light to indicate a distant lamp within a distant house. The sky above was rent by a few ragged clouds aglow from the moon, but otherwise was full of stars. Entranced by the view, I almost failed to catch the tiny glimmer of a lamp on the shore below, where the land steeply descended to meet the water.

Gelina had mentioned a boathouse. An outcropping of rock and the tops of tall trees obscured the view, but almost directly below me I could see a bit of roof and what must have been a pier projecting into the water, very small in the distance. I could also see at intervals a tiny flash of flame, coming and going. I listened more closely, and it seemed that each appearance of the lamp coincided with a soft splashing noise, as if something were being quietly dropped into the water.

I looked around trying to locate a stairway, and saw that a broad, descending path began at one end of the terrace on which I stood. I stepped carefully forward.

The path began as a paved ramp that doubled back on itself, then narrowed to a steep stairway that joined with another flight of stairs descending from elsewhere in the villa. The stairs narrowed into a trail paved with cobblestones that wound back and forth down the hillside beneath a canopy of high shrubs and trees. I quickly lost sight of the villa above and for a while could not see the boathouse below.

At last I rounded a comer and saw below me the roof, and beyond it the far end of the pier projecting into the water. A lamp flashed on the pier; there was a splash, and the lamp as quickly disappeared. In the same instant I felt my feet slip from beneath me and found myself skidding down the pathway, setting loose a spray of gravel that rained like hail onto the roof of the boathouse below.

I sat stock-still in the silence that followed, catching my breath and listening, wishing I had brought my dagger. The light did not reappear, but I heard a sudden loud splash followed by silence, then a noise in the underbrush below like the leaping of a frightened deer. I scrambled up and trotted down the pathway until it ended. Between the foot of the path and the boathouse there was a deeply shadowed patch of almost impenetrable darkness overhung by trees and vines. I stepped forward slowly, listening to the magnified sound of my own footsteps on the grass and the gentle lapping of water against the pier.

Beyond the circle of shadow, the boathouse and the pier were illuminated by full moonlight. The pier projected perhaps fifty feet into the water; it had no rail but was studded along either side with mooring posts. No boats were moored to it, and the pier was deserted. The boathouse was a simple, square building with a single door that opened onto the pier. The door stood open.

I stepped into the moonlight, towards the open door. I peered inside, listening intently, hearing nothing. A window high up in the wall admitted enough light to show me the coils of rope that lay on the floor, a few oars stacked beside the door, and the obscure implements that were hung on the opposite wall. Deep shadows filled the corners of the room. In the utter stillness I could hear my own breathing, but no one else's. I withdrew and stepped onto the pier.

I walked to the end, where the disk of the moon seemed to hover on the water just beyond the pier. The curving shore on either side was dotted with the lights of distant villas, and far away across the great flat water the lamps of Puteoli were like stars. I looked over the side of the pier, but there was nothing to see in the black water except the reflection of my own scowling face. I turned back.

The blow seemed to come from nowhere, like an invisible mallet swung from a black abyss. It struck my forehead and sent me staggering backwards. I felt no pain, only a sudden overwhelming dizziness. The invisible mallet swung out of the darkness again, but this time I saw it – a short, stout oar. I avoided the second blow by accident as much as by design – a staggering man makes an uncertain target. Flashes of colour swam before my eyes, but beyond the oar I glimpsed the dark, hooded figure who swung it.

Then I was in the water. Men who hire me sometimes ask if I can swim. I usually tell them I can, which is a lie. I shouted. I splashed. I somehow stayed afloat and desperately reached for the pier, even though the hooded figure waited there with the oar uplifted.

I reached for one of the mooring posts. My fingers slipped on the green moss. The oar swung down to strike my hand, but somehow I caught it in my grasp. I pulled hard, more to lift myself out of the water than to pull him into it, but the result was that my attacker lost his balance. An instant later, with a great splash, he joined me in the black water.

He came up beside me, struck me in the chest with his flailing elbow, and reached for the pier. I grabbed onto his cloak, frantically trying to climb over him onto the pier. Together we thrashed and struggled. Salt stung my eyes. I opened my mouth and sucked in a burning draught of saltwater. I lashed out at him blindly.

I think he knew that if he struggled with me I would only kill us both. He broke away and swam away from the pier, towards the overgrown shore beyond the boathouse. I clung to the slippery mooring post and watched him retreat like a ponderous sea monster, weighted down by his drenched clothing. His hooded head bobbed and retreated, bobbed and retreated. When he was safely far away I struggled onto the pier and lay gasping for breath. He disappeared into the shadows beyond the boathouse. I heard him climb out of the water, slipping and splashing, and then tearing through the underbrush.

The world was quiet again, except for the noise of my own laboured breathing. I stood up. I touched my forehead and hissed at the stinging pain, but I felt no blood. I staggered forward, my legs trembling but my head clear.

I should never have come to the boathouse by night, alone and weaponless; I should have brought Eco with me, and a lamp, and a good, sharp knife, but it was too late for that. I fished the oar from the water to use as a weapon-and hurried to the foot of the pathway. The way was hard and steep, but I ran all the way to the top, staring into every dark patch and swinging the oar at the invisible assassin who might be lurking there. The trail became stairs, the stairs became a ramp, the ramp opened onto the terrace, where at last I felt safe. I paused for a long moment to catch my breath. I began to feel the cold through my wet tunic. I hurried through the darkened house, shivering and still carrying the oar. I came at last to my room.