Marcia was about to put the Paired Code together. Her tiny study was packed and the atmosphere was electric. Even Nicko, who was not hugely interested in Magyk, was watching intently.

The tiny study window glowed an eerie red with the dimming of the SafeShield, but the study itself was bright with the light from a forest of candles dripping from a tall candelabrum set on Marcia's desk. Two books - The Undoing of the Darkenesse and The Darke Index - lay open on Marcia's desk. In the shadow of the books a small silver box and a tiny silver disc rested on a piece of purple velvet.

Alther had a bird's-eye view. To avoid the danger of being Passed Through, the ghost was sitting on the top step of a library ladder. He was watching the process with great interest. The use of the Paired Code was something Alther had known about in theory only. In his time as ExtraOrdinary Wizard both books that held the keys to deciphering the Code had long been lost. Marcia had found The Undoing of the Darkenesse in Aunt Zelda's cottage a few years back and she knew that somewhere within its pages lay The Great UnDoing - the legendary Anti-Darke incantation that practitioners of the Darke feared above all else. But its words were spread randomly throughout the book; to find them, the index to the book - The Darke Index - was required.

However, it was not that simple. Uncovering The Great UnDoing required more than merely using an index - it required using the correct pages of the index. This was where the Paired Code came in. In order to know which sections of The Darke Index gave the right sequence of page and word numbers in The Undoing of the Darkenesse, the Paired Code had to be read. Correctly.

And now that was about to happen. Under the rapt attention of Silas, Septimus, Jenna and Nicko - and the perching Alther - Marcia began to put the Paired Code together.

Marcia lifted out the Wizard Tower half of the Code and placed it on the velvet square on which its Pair - recently used to much less salubrious surroundings - lay waiting. She picked up the much smaller Manuscriptorium Code and placed its boss into the central indentation of the Wizard Tower Code. There was a brilliant blue spark and suddenly the Manuscriptorium Code was floating a fraction of a millimeter above the Wizard Tower Code. The Manuscriptorium Code now began to spin. Slowly at first, then faster and faster it went until it was no more than a flash of spinning light. There was a sharp click and the spinning disc stopped dead.

Everyone craned their heads for a closer look. The discs seemed to have fused into one and it was clear that the lines that radiated out from the Manuscriptorium Code joined up with some of those on the Wizard Tower Code. Each one of these led to a symbol. There was an awed hush. These were the symbols that would begin The Great UnDoing that would UnDo the Darke Domaine and set the Castle free.

Marcia got out her Enlarging Glass and peered at the symbols.

"Ready, Septimus?" she asked.

In his hand Septimus had his precious Apprentice diary, his pen poised at the top of a clean page. "Ready," he said.

The red glow from the failing SafeShield was beginning to fill the study, drowning out the candlelight. It fell on the smooth, blank page of Septimus's diary and cast threatening shadows across the room. Septimus knew it would not be long before the SafeShield was breached - it could happen any minute now, he thought. He waited, poised to write down the sequence of symbols that would lead them to The Great UnDoing. Why didn't Marcia begin reading out the symbols? There was no time to lose.

Jenna had guessed why but she hoped - desperately - that she was wrong. Unable to bear the suspense, she decided to test out her new Right to Know.

"But Marcia, how do you know which symbol to start with?"

Aware that she now had to answer all the Princess-in-Waiting's questions "truthfully, fully and without delay," Marcia looked up at Jenna and met her gaze.

"I don't," she said.

The little room fell horribly quiet as the implications of Marcia's reply sank in.

Simon pushed his way through the Darke Fog, terrified that at any moment a Thing would recognize him. He'd been lucky on the South Gate. The Thing on guard had done no more than stretch out a bony arm and pull him in without even looking at him. He knew he might not be so lucky next time. Simon wished that Lucy had not made him throw away his Darke Robes - "disgusting old things," she had called them. Right now he could have used them. Without their protection, the Darke Fog was suffocating - far worse than it had been in the Palace when it was still new. Now it had gained strength from all those it had overcome and it pressed down on Simon like a smothering pillow, closing off his ears and eyes, making each breath a huge effort.

Feeling as though he were walking underwater in lead boots, Simon struggled up Wizard Way, heading for the telltale red light of the of the Wizard Tower's failing SafeShield. As he waded past the Manuscriptorium he saw dim shadows of Things emerging and heading for the Great Arch, where they were gathering, waiting for the moment when the Barricade would fail. In a nightmare of slow motion Simon crossed to the other side of the Way and pushed on down the narrow lane that ran around the Wizard Tower courtyard wall. He was heading for the ExtraOrdinary Wizard's Hidden side gate, which was not visible from the outside and so, he hoped, would not attract the attention of any Things.

When Simon arrived at the lintel that marked the presence of the Hidden gate his head was spinning and he felt as though the Fog was inside his brain. He longed to rest his heavy limbs, to lie down for a moment, just a moment . . . he leaned against the wall and felt not stones but wood and a latch beneath him. Slowly his eyes closed and he began to slide to the ground.

Strange things happen in the dying phases of a Living SafeShield. The separate components begin to make their own decisions. So when Simon slid down the Hidden gate it Knew it needed to let him in. It swung open and he half rolled in. The gate did a nifty flick, pushed him inside and closed as fast as it could. A few tendrils of Fog curled in with him but stopped when the gate became one with the wall once more.

The clear air inside the Wizard Tower courtyard soon woke Simon up. He got shakily to his feet and took a deep breath. He looked up at the Tower rearing high above him, almost dark now - the only light the red of the dying SafeShield - and he felt quite overawed. Shakily he headed across to the wide marble steps that led up to the silver doors that guarded the Tower.

Once again the Living SafeShield recognized help when it saw it. The tall silver doors opened noiselessly and Simon, heart beating fast, stepped into the Great Hall. As the doors swung closed, Simon took stock. He could hardly believe that he was actually inside Wizard Tower. For so long he had dreamed that one day he would set foot in the Tower and rescue it from danger, and now that that was exactly what he was doing, it did not seem real.

But things in the Wizard Tower had changed. Simon had not been in the Great Hall since he was a boy. He remembered it as a bright, joyful place buzzing with Magyk, with beautiful pictures flitting across the walls and a fascinating floor that wrote your name when you stepped on it. He had loved the mysterious smell of the Magyk and the sharpness of the air, and the purposeful hum of the gently turning silver spiral stairs. And now it was all very nearly gone.

The lights were low and dull, the walls dark, the floor blank and the silver spiral stairs were Stopped. Every-thing was winding down. Shadowy figures of Wizards and Apprentices were scattered about the Great Hall, the younger ones wandering anxiously to and fro, the older ones slumped with exhaustion as they concentrated on the uphill struggle of adding their tiny piece of Magykal energy to the SafeShield.

Hildegarde stepped out of the shadows. Pale and drawn, with dark circles under her eyes, she watched Simon walk to the stairs. She did not stop or question him. It was a waste of energy. If the Tower had let him in, he was here for a reason. She just hoped it was a good one.

Simon ran up the Stopped stairs. Up through the darkened floors he occasionally heard a weary murmuring of a Magykal chant, but mostly he found nothing but silence. Outside he could see the red light fading fast and he knew that once it had gone, the Darke Domaine would enter the Wizard Tower. Simon did not know how long that would take but he guessed it was minutes rather than hours.

On the twentieth floor he jumped off the stairs, ran along the broad corridor that led to the ExtraOrdinary Wizard's purple door and threw himself against it.

Inside the study Marcia was dictating the symbols that the lines on the Manuscriptorium Code had picked out. She had decided that the only thing to do was to begin with each one in turn. There were forty-nine matches. This meant there were forty-nine words in The Great UnDoing - and forty-nine possible beginnings, of which there was no way of telling which was the right one. As The Great UnDoing was an ancient incantation, Marcia knew that it would not necessarily make any sense, so there would be no clue as to what might be the first word. It was a huge risk, but she had no alternative. It was just possible they might find the correct order right away. It was the only chance they had and Marcia knew she had to take it.

And so she was rapidly dictating. "Zero, star, three, Magyk, labyrinth, gold, Ankh, square, duck - yes, I did say duck - two, twin, seven, bridge - oh!" Marcia looked up suddenly.

"My door . . . it's let someone in," she whispered. "There's Darke on them. From outside."

There was a sharp intake of breath. "I'll go and check it out," said Silas, heading for the study door.

"Silas, wait." Alther got up from his perch. "I'll go. Bar the door when I've gone."

"Thank you, Alther," said Marcia as the ghost quickly DisComposed himself and walked through the door. "Now, where were we? Oh, drat, I don't know. Septimus, I'll start again. Zero, star, three, Magyk, labyrinth, gold, Ankh, square, duck, two, twin, seven, bridge, spiral, four, ellipse, plus, tower - Alther, is that you?"

"Yes. UnBar the door please, Marcia. Quickly. I have someone to see you."

Everyone exchanged questioning glances. Who could it be?

Alther ushered in Simon to a stunned silence. "Before you say anything, Marcia, this young man has some important information. He knows where to begin."

"He does?" Marcia frowned. "Alther, there are other Invocations on this Code - and some are downright dangerous. How can I be sure he will tell me where to begin the correct one?"

Septimus, Nicko and Jenna looked at each other. Other invocations? So Marcia was gambling that they would reach the right one first. Things were even worse than they had thought.

"I've known him since he was born," said Alther. "I believe you can trust him."

"You can trust me. I promise," said Simon quietly.

Marcia looked at Simon. He was soaking wet, trembling with cold, and there was desperation in his eyes - a desperation that mirrored exactly what she was feeling that very moment. She made her decision.

"Very well, Simon," she said. "Would you show us where The Great UnDoing begins?"

And so Simon found himself somewhere he had never thought possible. At the top of the Wizard Tower, sitting at the desk of the ExtraOrdinary Wizard, surrounded by fabled Magykal books and objects - including, he noticed, his very own Sleuth. And now, watched by his father and his youngest brother, he was about to tell the ExtraOrdinary Wizard something that would save the Castle.

"The starting point is given in the index of The Darke Index," he said.

With trembling hands Simon picked up the book. For a moment it felt like an old friend, until he remembered that, in truth, it was an old enemy. The countless cold, lonely, and sometimes terrifying nights he had spent reading it came back to him and he remembered the last time he had held it when, in an early attempt to give up the Darke, he had stuffed it into the back of a cupboard and locked the door. He had never dreamed that the next time he held it he would be in the Wizard Tower.

Gingerly he opened The Darke Index at the inside of the back cover. Muttering a short incantation, he ran his finger across the well-worn endpaper and, as he did so, letters began to appear beneath his fingers.

An irritated tut came from Marcia. A simple Reveal - why hadn't she thought of that?

Beneath Simon's moving finger an alphabetical list began to Reveal itself. His finger slowed at G and everyone waited, but The Great UnDoing was not listed. Simon's finger slowed at the T but The Great UnDoing was not there. A palpable lack of confidence began to fill the little room and when Simon reached the letter U his hand began to shake. Suddenly "UnDoing. Great. The." appeared. Smiling with relief, Simon handed the Revealed index to Marcia.

"'UnDoing. Great. The. Begin with Magyk, end with Fyre,'" she read aloud. "Thank you, Simon."

Simon nodded. He did not trust himself to speak.

Marcia sat down. She put her spectacles on and opened The Darke Index. "Now, Septimus, read the symbols out to me again, beginning with Magyk. Slowly, please."

And so Septimus went through the list. At each symbol he paused while Marcia quickly leafed through the pages, grubby and grease-stained from Merrin's sticky hands. Each page had one of the symbols at the beginning of the text. At the foot of the page - looking to the casual observer like page numbers - were two numbers. Marcia noted down the numbers, then said briskly, "Next." It seemed to take forever but it was only a matter of minutes before Marcia had a column of forty-nine pairs of numbers.

Marcia handed Septimus the numbers and then she opened The Undoing of the Darkenesse.

"Read the numbers out to me please, Septimus."

The red glow suffusing the study went out like a light. There was a collective gasp.

"SafeShield's out," Marcia said grimly.

* * *

Far below, the Barricade smashed to the ground and the first Thing walked across it, into the Wizard Tower courtyard. Twelve more followed, along with a stream of Darke Fog.

At the top of the Tower Septimus read out the first number of the first pair. "Fourteen."

With urgent fingers Marcia flipped the thick pages of The Undoing of the Darkenesse to page fourteen.

Septimus read out the second number of the first pair. "Ninety-eight."

As fast as she could Marcia began to count along the words on page fourteen until she reached the ninety-eighth word.

"Let." It seemed a very small word for all the trouble finding it.

And so, agonizingly slowly, Marcia began to put together The Great UnDoing.

Outside the Wizard Tower, on the topmost marble step, a Thing reached out a long bony finger and pushed against the tall silver doors. They swung open like shed doors left unlatched in a summer breeze. The Thing walked into the Wizard Tower and the Darke Domaine tumbled in after it. The lights went out and someone screamed. In the shadows of her tiny office Hildegarde was suddenly certain that her little brother, who at the age of seven had disappeared during a Do-or-Die exercise in the Young Army, was outside the door. She ran to open it and the Darke Fog rushed in.

Things streamed in across the threshold, bringing the Darke Domaine with them. They milled around, squashing the dying floor beneath their feet, watching Wizards and Apprentices slump to the ground. As the Darke Fog began to fill the hall, the Things wandered across to the Stopped stairs and began to climb. Behind them the Darke Domaine moved slowly up through the Wizard Tower, filling every space with Darkenesse.

At the very top of the Tower, Marcia had in her hands a piece of paper with a string of forty-nine words on it, which formed, she sincerely hoped, The Great UnDoing. She and Septimus were running up the narrow stone steps to the Pyramid Library with Alther following in their wake. They flung themselves through the little door and Marcia hurried over to the window that led outside. She turned to Septimus.

"You really don't have to come," she said.

"Yes, I do," said Septimus. "You need all the Magyk you can get."

"I know," said Marcia.

"So I'm coming with you."

Marcia smiled. "Out we go then. Don't look down."

Septimus looked neither down nor up. Focusing only on the hem of Marcia's purple cloak, he followed her up the stepped side of the golden Pyramid. Alther flew slowly behind.

And so, for the second time that night, Marcia stood on the tiny platform at the top of the golden Pyramid. For some reason, she wasn't sure why, she took off her purple python pointy shoes and stood barefoot on the ancient silver hieroglyphs that were incised into the hammered gold top. She waited for Septimus to join her and then together, in voices that cut through the Darke Fog, they began the forty-nine-word incantation of The Great UnDoing.

"Let there be . . ."

Far below, the leading Thing poked its finger lazily at the great purple door that guarded Marcia's rooms. Twelve Things stood behind it expectantly, waiting to take over their new abode. The door swung open. The Thing turned to its companions with what was possibly a smile. They stood, savoring the moment, watching the Darke Fog tumble in and swirl around Marcia's precious sofa.

At the top of the golden Pyramid, Marcia Overstrand, ExtraOrdinary Wizard, and her Apprentice, Septimus Heap, spoke the last word of The Great UnDoing.

With a great crash Marcia's door slammed in the Things' faces. A loud whirring ensued - the door Barred itself and, for good measure, sent out a Shock Wave. Thirteen Things screamed. A scream of thirteen Things is not one of the most harmonious sounds, but to Septimus and Marcia, teetering at the top of the golden Pyramid, it was the sweetest thing they had ever heard.

And then they saw the most beautiful sight they had ever seen - the Darke Fog rolling away. Once more they saw the Castle they loved - its higgledy-piggledy roofs, its turrets and towers, its crenulated battlements and tumbledown walls, all outlined against the pink sky of the dawn of a new day. And as they watched the sun rise, dispelling the shadows that lurked below, the first heavy snowflakes of the Big Freeze began to fall. Marcia and Septimus smiled at each other - the Darke Domaine was no more.

Some minutes later, a broadly smiling Marcia was ushering everyone into her sitting room, busily opening the windows to get rid of the dank smell of the Darke. Jim Knee was curled up in his usual place on the sofa with Jillie Djinn beside him, just as Marcia had left them. But there was something about the Chief Hermetic Scribe that made Marcia hurry over to her.

"She's dead!" Marcia gasped. And then, much more dismayed, she cried, "She's dead on my sofa!"

Jillie Djinn was slumped backward, mouth a little open, eyes closed as if asleep. Her body was there, but she herself had clearly gone - whatever it was that had been Jillie Djinn was no more. The Great UnDoing had been her undoing also.