'Sir Thomas, sire... You need to get up.'

Sir Thomas Kartzela jerked his eyes open, his dreams dispersing like smoke. It was early, and the grey morning light leaked into his bed chamber through the narrow east window. Somewhere a bell was clanging, an ugly, urgent sound.

The great black key that hung from his neck was cold against his skin. He touched it with his left hand, reflexively, as he did whenever he woke.

At the far south of the principality of Kuek, over the chasm and thundering river that formed its southern-most border, was a stone bridge. It was built three hundred years ago, by hands unknown, and it spanned the gulf, far above the raging water. It was wide enough for three knights to ride abreast, although it had no walls to prevent a rider from falling; and it was held up by pillars that seemed impossibly narrow, but nevertheless still supported its moss-covered bulk.

'Sire, out there. There's something you need to see...'

The voice called through his door. The bell kept up its relentless noise.

The old knight pushed the blankets away and sat up in his bed. He swung his legs out, and his bare feet met the smooth wooden floor.

The north end of the bridge had a gatehouse. It was as old as the bridge; and it too was made of smooth stone blocks, their joins almost invisible. The gate itself was wood, heavy solemn oak, a pair of huge doors studded with metal.

The south end of the bridge had no buildings, and beyond it was a track that petered away into scrubby moorland. The mists hid whatever was beyond, and humanity did not go there. It was the edge of the world, and this bridge was the sole place where you could reach it.

Sit Thomas pulled on tunic and boots, the key cool against his skin. He picked up his sword belt, and his right hand closed around the hilt, like it always did. Then he strapped the belt around his waist, threw his black cloak over his shoulders, and opened the door to his room.

His family had ruled these lands for as long as they had held records. The legend went that the first King had gifted this gatehouse to his most loyal knight, and told him to keep watch. If that was true, it was a meagre gift: out here, where the rain streaked from the grey sky, and the purple flowers bobbed in the brutal cold south winds, humanity didn't thrive, they clung to the land. The Kartzela family motto was simple: 'endure'. And that was what they had done, generation upon generation, their little family castle not much larger than the tumbledown huts of the serfs who scraped along beside them. Even the Kings had fallen, usurped by warring princes; but the bridge, the gatehouse, and the Kartzelas remained.

'Sire, you need to come to the gatehouse.'

It was Gwynn, the youngest of his garrison of four. He was barely an adult, his beard not much more than fluff. He was nervous, his eyes darting around like minnows, beads of sweat on his forehead even in the cold morning air.

Sir Thomas nodded. 'Show me.'

They set off down the stairs, down to the stables, lighting their way with tallow candles that flickered in the draughts.

Sir Thomas was the last of his line. His mother had died bringing him into the world, and it had broken his father, who had passed away a mere decade later. It had been his family's custom to send the eldest son to the city of Hallen, to find a wife, a minor noblewoman who would thrive in the wild loneliness of the south, so that the family could continue and they could endure; but James had been the only surviving child and he had taken up his father's spurs at ten and had no time or ability to travel. And now here he was, at fifty six, as grey and seamed as the stone that he ruled over, riding to the ancient gatehouse that was his heritage.

'I came to get you straight away, sire. I rang the bell and woke the others. I don't know what it is...'

The boy was babbling, riding alongside.

'Hush, Gwynn,' the knight said.

The rolling morning mists were dispersing in the weak sun, exposing earth and stone and grass. Ahead of them the bell clanged with dull urgency, and then stopped, the last stroke muffled in the damp air.

'Hail Sir Thomas,' Roderick shouted from the gatehouse. His sword was out, held up in salute.

'Hail Roderick,' Sir Thomas replied. He rode up to where the man was standing, so that he could speak without raising his voice. 'What's the reason for the alarm?'

Roderick was nearly as old as the knight, and had served him since childhood. The two of them had grown old together, and Sir Thomas trusted the man with his life.

'We have a visitor, sire,' Roderick replied.

'On the bridge?'

'Yes sire. He's standing at the centre.'

Sir Thomas nodded, his grey eyes on the door. 'Thank you, Roderick. Gwynn, ride to the guard tower at Helleth, as hard and fast as you can. Let them know. Go. Go!' As soon as the boy had left, he turned to Roderick. 'I want my lance, cuirass, and shield. Prepare to open the gates.'

Roderick shouted to the other two men to get the knight's armour. They blinked and then did as they were told, still stupefied from being recently woken by the bell.

Sir Thomas's family crest was a sable sword and key, crossed, over a grey background. This was painted on his shield, the black symbols huge on the heavy wood. His armour was simple and tough, designed for war, not show. He had drilled in it every day for his entire life, and it felt like a second skin. It was oiled and cold, the same grey as the sky, the same grey as his shield. He threw his black wool cloak over his shoulders, and took the key from round his neck, and passed it to Roderick, and then remounted his horse. Roderick handed him a lance, as unadorned and brutal as everything else he owned. The men ran to the lock, and pulled the huge doors open, grunting with the strain.

Sir Thomas Kartzela rode a few steps onto the bridge, lance lowered, hood down against the light rain. There was indeed a visitor, standing at the centre of the bridge.

Roderick's men pushed the gate closed behind him and then locked it, as they had practised together endlessly.

'Hail, stranger,' the knight shouted. 'What's your business here?'

The stranger looked like he'd been carved from a kinder sky. His cloak was the pure blue of a summer morning, edged with the gold of sunrise. His skin was the ultramarine of a clear, autumn evening. His sword was the orange of the clouds at sunset. His hair was black, and twinkled with tiny silver stars. He stood, on foot, alone, his sword sheathed.

'Hail, sir gatekeeper,' the stranger replied. 'My name's Khate. I've got a message for your King.'

His voice was low, and clear.

'There are no more Kings,' Sir Thomas replied. 'This land's called Kuek, and it's ruled by my Prince. What do you want to tell him?'

The stranger, Khate, nodded. 'I understand – a lot has changed. Then send this message to your Prince, and he can decide what to do with it. The message is this: "the jailer is nearly ended, and you will be free." Goodbye, Sir Thomas Kartzela.'

He bowed, and turned, and walked back, across the bridge, out into the mists at the edge of the world.

When he had disappeared, Sir Thomas turned, and rode back through the gate.

Gwynn returned from Helleth at nightfall, alone. The soldiers there had laughed at his tales of blue men walking from outside the world, but had agreed to send someone in a week's time. Sir Thomas didn't blame Gwynn. Instead, he set up a rota whereby he and Roderick would take turns to lead the watch, so that if the stranger returned he would be ready. Then, when the old knight wasn't at the gatehouse, he was combing his family's books, looking for clues about the stranger.

He was deeply troubled. He wasn't surprised that the soldiers hadn't believed Gwynn; he wouldn't have either. This was the very edge of everything. No one knew what was beyond the bridge, but whatever it was, it didn't involve blue men.

The days turned to weeks. He sent Roderick to ride to the Prince's palace at Hallen, because he felt that Roderick might have a chance of not being laughed out of court. He himself couldn't abandon his post, and so he took up residence in one of the cold stone rooms of the gatehouse itself. There, in that room which had no tapestries or glass in the windows, he was woken every morning by the crows chattering, and fell asleep to the sound of the sheep calling mournfully from the nearby pastures.

Two days after Roderick had left, Gwynn found what he thought might be the answer. It was an ancient scroll, forgotten under a pile of old books in a cellar in the castle. He brought it to the gatehouse in high excitement. When Sir Thomas unrolled it, the vellum was still smooth, and the writing was tiny and legible.

It was a deed of property, signed with an ancient seal, and it told his family legend.

The castle and the lands were indeed a gift from the King. The region was known as Kartzela, and his ancestor, that first knight, took the name. That man was also named Thomas, which was not a surprise, as the name was handed down from father to eldest son. The family was exempted from taxation and draft, and allowed to rule their land as they wished with no interference from even the Crown. It almost carved out the tiny region as an independent country. There were two stipulations: the first of which was to man the gatehouse and guard the border in perpetuity.

Sir Thomas was in the room at the top of the west tower, looking down at the bridge, reading by candle in the early evening. Well, he and his ancestors had been good for that, so far.

The second was that if the Kartzela line was ever to end, the King must know; and to that end, a record of every birth and death should be sent to his court in faraway Hiria.

Sir Thomas scratched his grey beard, and considered this. These days Hiria was under the control of another prince. His family must have stopped sending that information when the kingdom had fragmented after the death of the final king, a hundred years ago. And here he was, the last knight in Kartzela, with no way to fulfil his family's obligation.

He looked up at the grey sky. It would soon be night.

Something else worried him too, and it wormed at him all that time when he sat or lay in the cold, draughty rooms of the gatehouse, and even this document gave him only a partial answer for it.

How had the blue stranger known his name?

Roderick was back, a week later, having taken the message to the Prince. It had been ignored. Again, Sir Thomas was not surprised. What ruler, safe in the warmth and luxury of his castle, would worry about crazy stories from an old man about a bridge at the edge of the world?

And so the weeks turned to months, and he kept his vigil in the gatehouse, and nothing happened.

His health deteriorated. The winter was cold and unforgiving, and he couldn't keep warm and comfortable in the rooms in the gatehouse. He missed his bed chamber in his castle, but something in him pushed him to stay out here, right at the border. He knew from the deed that this was his destiny, that his family had been given this task and it was up to him to see it end. He would endure.

In the early spring he fell into a fever. It crept up on him, like the rain that drummed on the battlements above, and dripped on the glassless sills, splashing into his room; he found himself endlessly cold and tired, no matter the weather or how much he slept. Then, when the days were getting longer and the first tiny purple flowers appeared, on a morning when he was looking across the empty bridge with nothing but clouds and blocks of stone as his companions, he was too weak to stand.

He collapsed where he stood, the world swimming around him; and in his delirium, he was convinced that he was at the centre of both everything and nothing; that all made sense, and there was no mystery. But the truths fled from him when he tried to catch hold of them, like flitting fish in a lake that he thrashed in.

He found himself lying on the stone battlements above the gate. He clawed his way to his makeshift quarters, mumbling and drooling, scraping his flesh on the naked rock of the walls, only partly aware that his dignity would be gone if there were anyone to see. He collapsed into his bed, a rough palette of furs and blankets, and he surrendered himself to sleep.

Some time later, he was sensate again. He woke to find himself coughing, as the wan morning sun filled his chamber with grey light, just like that day months ago, when the stranger first came. When he opened his eyes, he saw the wooden floor, the stone walls, and the grey sky. There was an iron bowl full of food, and another full of waste. He had no memory of either of them.

He sat up. The black key still hung from his neck on its leather cord. His sword was next to him. The two symbols of his heritage were there, and that meant something; his left hand reached for the key, and his right went to the sword, and the feel of the iron comforted him. He wondered what had happened, how long he had slept. Hours? Days? He coughed again, his lungs and throat dry and rough.

The bell clanged, tuneless urgency. It took him a few seconds to remember what it meant. He pulled himself to his feet, leaning on his sword. He realised that he was naked, so he dragged on what clothes he could find, finishing it with his black cloak. He couldn't dispatch the stink that way, but it would have to do. He pulled the hood over his head to protect himself from the rain.

The stairs were a puzzle, every step a new challenge. Eventually he found himself outside, bare feet on damp earth, where Gwynn was hammering the bell.

'You can stop now, Gwynn.'

He was amazed at how hard it was to talk. Every breath was hard won, every syllable a victory. The boy spun towards him, eyes wide.

'Sire, you should sleep!'

'I've done quite enough of that.' Even he could hear the lie in that sentence, but he pressed on. 'What's the alarm for?'

'The stranger's back, sire.'

'Let him through. I wish to speak to him.'

He'd trained his men well. They'd drilled together for a lifetime, run through this mists, sparred in the rain. So, Gwynn didn't argue, although there was fear in his eyes. He held out his hand, and Sir Thomas gave him the black key. He unlocked the gate, and pulled open one of the doors.

Khate was standing right at the threshold. He didn't cross it; instead, he waited while the old knight, supported by Gwynn, walked to meet him. Sit Thomas tried hard not to cough, but it was no good; his body was racked with them, and his breath came in wheezes.

'Hail, Sir Thomas,' Khate said.

Sir Thomas knew he didn't have much time left. He wanted to know the answers before he died, and he wasted no breath on greetings.

'I read the King's command,' he said. 'My family was entrusted with this land. The King knew something about you. What are you?' and then, a flash of inspiration: 'Your people were imprisoned. My family was set to keep you locked away.'

Khate shook his head. 'You look like him, Like the original Sir Thomas. You have his determination. Three hundred years haven't softened that obstinacy. I used to joke that he was cut from the same stone that this bridge was made from. Seeing you now, I don't think that it was a joke.'

Sit Thomas coughed again; it took all his strength to stand, even with Gwynn's help. 'My family was made to endure. And we have.'

'Yes, you have. And now your line is nearly over. No, Sir Thomas, you were not my guard. You were the jailer of the people on your side of the bridge. Your family's time is gone, and your obligation is fulfilled; and that means you and your people have marked their punishment and are free to return.'

'Return?'

'Come with me, both of you. I'll show you, and you can die in peace, Sir Thomas.'

And the three of them walked south, over the bridge; and behind them they left the door open forever.