Belisarius of Constantinople, who arrived in Britain knowing nothing of Isolde's Menologium, never meant to come to Lindisfarena. After a long journey across the Mediterranean from Greece, his precious books wrapped in pigskin and stacked in sturdy trunks, he had sailed on a Frankish ship from Massilia to the port of Brycgstow. He had come to meet a trader called Theodoric, with whom he had worked many times before, and to deliver his antique books. His sojourn in Britain should have ended where it began, in Brycgstow. And it would have if not for chance – and his own reckless nose for adventure.

After disembarking he had an hour or two to spare before he was due to meet Theodoric, and he wandered through the town.

Brycgstow was a crude place. Built by Germans on no discernible civilised template, the town's wooden buildings clustered like turds in a cow field and its roads straggled like sheep tracks. The dock area was crowded, and the strand heaped up with goods – no wharves or warehouses here, you just dumped your cargo on the filthy beach. Belisarius kept a sachet of Syrian spices hanging at his neck to keep out the stink of blood and piss and dung, and he tried not to wince when his boots were spattered with filthy mud. On travelling outside the empire he always carried spare shoes.

But Brycgstow greeted ships from all the petty kingdoms of Britain, from Frankia and the northern countries, from the Moors of Iberia and the Goths of Italy, and of course from the East Roman Empire. It was a model of the whole world in miniature, Belisarius thought, as all ports were.

And Brycgstow was famous across Europe as a slave market. The west of Britain was an intersection between the aggressive, squabbling German kingdoms and the older British domains; the endless wars were good for the slave trade. Many of the cowed captives in their pens were Latin-speaking Britons who still thought of themselves as Roman, and would come laden with awkward aspirations. But the various breeds of German were not above selling their own kind when they got the chance. They all looked alike to Belisarius, whose own family were Greeks. He kept few slaves himself; he found slavery distasteful, and he was relieved to stroll away from the dock area, deeper into the town.

He walked through a quarter of manufactories, passing a cobbler's, an iron worker's, a silversmith's. He reached a small market where animals swarmed around stalls piled high with meat and stunted-looking vegetables and fruit. The people were a rough lot, but they seemed healthy enough, tall, many of them blond, and with good teeth. Many wore striking brooches at their shoulders, and necklaces of beads or silver tokens strung across the chest. Men and women alike wore their hair tightly bound up on their heads.

Belisarius stood out from the crowd, with his clean-shaven Greek looks, and his modest but good-quality clothing. He had no fear, however. East Romans had been trading here for centuries, ever since the severance of this old province from the collapsed western empire. And Belisarius's father, who had served as a soldier under the great emperor Constantine V, had raised his sons so that they were capable of defending themselves. Aged forty but still fit, Belisarius met any challenging glare frankly.

Indeed, far from fearful, Belisarius was curious. A seller of books, he fancied himself a writer, and in his travels, from Germany to Iberia, Persia to Britain, he had recorded his observations – a mosaic of his times, as he thought of it. For now his project was just a heap of disparate notes, jottings and sketches. But when he got time, when he settled down in his Constantinople town house with his wife and boys, he would pull it all together into a coherent narrative. Even a tale of a town of mud hovels as unprepossessing as this might have a certain appalling fascination for a matronly reader of the east…

It was with his head full of such musings that he came upon the church, and the trial.

Curious, he paused by the open door. Compared to the ecclesiastical glories of Constantinople, he would barely have recognised this as a church at all. It was stone-built, however, to a sound rectangular plan, and the lichen on the rain-streaked stone told of age. But the small, dark, crowded space within, which had a sharp, hot stink like a forge, was host to no ceremony Belisarius recognised.

Near the altar a fire was burning in a brazier, and a rod of iron lay across the fire, so that it was glowing red-hot. A dark, low-browed fellow waited by the brazier, looking distinctly ill at ease. A priest took a cup of water and walked up and down lines of waiting men, sprinkling their foreheads as they grunted their way through Germanic prayers.

Then the priest donned a heavy glove, lifted the iron from the fire, and laid the bar on a wooden post which scorched with a hiss. Evidently it was hot enough. The priest nodded to the dark man and stood back.

The dark man, the victim, raised his bare hand to the iron. He looked around at the others, even at the priest, with evident contempt – but then his eyes lit on Belisarius, and widened a little, as if in recognition. Belisarius had no desire to be drawn into this personally. But he could not withdraw from the spectacle.

The dark man closed his hand around the rod. There was a gruesome sizzling sound, and a smell like burned pork. The men in the church, perhaps this victim's accusers, flinched and turned away. But the dark man stood defiant, glaring at them, holding the bar aloft with his smoking hand. Admirably, he made no sound. Then he marked out paces, counting deliberately, walking between the two lines of men. After nine paces he opened his hand. The metal stuck to his burned flesh, pulling it away from his palm, before the rod dropped to the floor with a clatter.

The priest wrapped the wounded hand in a grubby cloth. The accusers, solemn, began to file out of the church. Belisarius understood little of the ragged tongues spoken by the Germans, but he picked out one phrase, intoned gravely by the priest: 'Three days.'

And when the dark man walked out of the church, to Belisarius's dismay, he approached the Greek directly. He looked perhaps thirty, and his small face was dominated by thick black eyebrows that underlined a low forehead. He was smaller than Belisarius, his clothes might once have been smart but were now much repaired and shabby, and he was pale and slick with sweat. He raised his bandaged hand, and said in accented Latin: 'My name is Macson. I know you.'

'I'm afraid I don't-'

'Help me.' And he fainted dead away, crumpling at Belisarius's feet.

Gudrid had always been fascinated by the old family legend of her ancestor Ulf the Wanderer and Sulpicia, the British girl he had loved and lost, and the strange prophecy of the Roman Christ-god which Ulf had remembered – and then forgot, and so, in a way, lost too. Perhaps it was because her own life was so drab that she was drawn to a tale of doomed love in the past.

But it was not until the chance arrival of a British slave that she had the opportunity to do anything about it.

She was working alone that day, in a patch of forest high above the fjord. The trees here had already been felled, and Gudrid's job was to strip away branches from the trunks, which would then be hauled down the hill. She worked with a will, and the iron blade of her axe, coated with whale-oil, flashed as she drove it into the wood. She was twenty years old, tall and strong. This wasn't a woman's work – but then, as her husband Askold had once told her in his cruelly indifferent way, a wife who had failed to deliver a single son was barely a woman at all.