On Lindisfarena it was a late May morning, in the monastery's study period, when Elfgar and his black-souled cronies came for Aelfric. That was the chance unpleasantness that began her own true involvement with the Menologium.

For Belisarius, bookseller of Constantinople, it was chance too, an encounter with an ambitious Briton in a southern port and an ordeal by fire, that lured him to Lindisfarena.

And Gudrid was drawn here all the way across the sea. She shouldn't really have come at all. But while her father and her husband came for gold, she came in search of a legend of love.

None of them would have been there, none of their lives perturbed, if not for the promise of the Menologium, with its tangled threads reaching from lost past to furthest future. None of them would have been there but for the Weaver.

The day started well for Aelfric.

She walked barefoot across the dewy ground to the church. The monks' blocky shadows as they padded over the grass around her, the hems of their woollen habits rustling. The second equinoctial hour, when the monks were called for the night service, Matins, was usually a gruesome time to be stirred from your cell. This morning, though, it was warm and not quite dark, for midsummer was approaching, and the island of Lindisfarena was so far to the north of the world that even now a little light lingered in the sky.

They all crammed into the church. Immersed in the stink of damp wool, the monks signed, mimed and gestured to each other busily. But not a word was spoken, for the rule of Saint Benedict, whose instructions governed every aspect of the monks' lives, was that the first spoken words each day should be in praise of God. The candlelight evoked deep colours from the tapestries and friezes on the walls, and from the silver and gold that adorned the shrine of Saint Cuthbert. The wooden church was a place of sanctuary, of warmth – for, despite unpleasant worms like Elfgar, this was indeed Aelfric's family now.

Led by the abbot, the monks began their chants. Aelfric tried to deepen her voice, but she sang with gusto. She had been taught that the chants were devised by an Arch Cantor based in Rome itself. It was a wonderful thing to imagine all of Christendom, all across Europe, singing the same beautiful songs.

But even as the brothers sang, Elfgar watched Aelfric, his gaze as heavy as lead.

She had spotted his rapacious look as soon as she had landed on Lindisfarena. It was a look she had not expected to encounter here, among the monks of the Shield Island. Perhaps he could smell the stink of a woman on her. But she saw the way others, even those older than herself, cowered from Elfgar's gang.

A pilgrim might come away believing that the oblates, deacons and novices laboured at their daily duties here under the stern but holy eye of the abbot, that their bodily needs were tended to by Domnus Wilfrid who made sure they were fed and clothed, and their souls guided by their tutors, such as Dom Boniface who watched over Aelfric herself. But in the underworld of the novices and deacons there was another power, wielded by the likes of Elfgar. Monks were humans too, and in some ways the monastery was just like the thegns' halls where Aelfric had grown up, Elfgar like a bully among the athelings. Aelfric didn't know what he wanted, but she knew her time with him would come.

And what she really feared was losing her secret: that Elfgar might find out that her name wasn't Aelfric at all but Aelfflaed, that she wasn't a young man but a woman, and that she shouldn't be here at all in this all-male house of God.

When Matins was over, the monks were released for a bit more sleep before Prime, the first of the day's six services. But that morning Aelfric didn't want to go back to bed. As the monks filed out of the church the dawn light was enticing – a deep rich blue that had a trace of purple in it, she decided with the eye of one who was learning to master colour in her inks. On impulse she ducked away from the others and cut south towards the shore. She walked briskly, swinging her arms and pumping her legs, relishing the crisp sea air in her lungs and the feel of the blood surging in her veins.

At the sea she walked into sharp-cold water up to her ankles. Gritty sand, speckled with bits of sea coal, slid between her toes. She was seventeen years old, and she had grown up hunting and play-fighting every bit as hard as her brothers. She longed to throw off her heavy habit and run, naked, into the ocean's cold water. But that, of course, was impossible; this moment of paddling must be enough.

With her ankles in the water, her habit hitched up around her knees, Aelfric looked back at the monastery she had made her home.

The island of Lindisfarena was round, like the shield from which it took its name (lind was an old British word), and small enough that you could walk across it in an hour. A sandy spit ran off to the west, which the monks called the Snook – like the arm of the warrior who bore the shield-island. Lindisfarena was only sometimes an island, however. A causeway, a trail of sand and mud flats, linked the western end of the Snook to the mainland, but it was drowned for five-hour periods twice each day. Aelfric could see wading birds pecking for food along the length of the causeway, and seals gambolling like hairy children in the shallow waters.

The monastery itself was unassuming. Within a low wall huddled the cells of the monks, crudely-built domes of stone that everybody called 'beehives'. Aelfric herself shared a wooden-walled dormitory with other novices, smooth-faced boys, mostly the sons of thegns, too dull-witted even to notice that they were living with a woman. More square-built buildings clustered, a refectory and kitchen, an infirmary, a hospitium for any guests – and of course the library and scriptorium. A thread of smoke rose up from a kiln for bread-making.

From here it looked austere, frugal. Tiny and remote the island was, modest its monastery might look, but Lindisfarena was one of the most famous Christian sites in the world. It was to its off-shore isolation that King Oswald of Northumbria had summoned Saint Aidan of lona to convert his pagan people, more than a hundred and fifty years ago. From that beginning Northumbrian Christianity had become so strong that where once Rome had sent missionaries to a pagan Britain, now Northumbrian missionaries worked in the lands of the Franks and the Germans.

And it was rich. Aelfric mused that if the church's wooden walls could be turned to glass you would be dazzled by the gold and silver revealed within. A century ago Lindisfarena had become the shrine of Saint Cuthbert, and pilgrims had come here ever since, all bringing money, even if only a penny or two each.

In this remarkable place, here was Aelfric, hiding her sex so she could read a few books.

The light was brightening. She realised she had no idea how much time she had already wasted, standing here in the sea. But then today was a fast day, one of no less than two hundred in the year, and she could always give up her meal-times to work in the scriptorium. She plodded out of the water and ran through the thick sand back towards the monastery.