The night wore on in the room behind the Big Red Door, its occupants sleeping fitfully on the odd assortment of cushions and rugs. They were rudely awoken twice by Thunder - who was not named just for the stormy color of his coat - but after protests and much fanning of the air, everyone managed finally to drift off once again.

Jenna had appropriated her old box bed in the cupboard, which still had the rough, threadbare blankets of her childhood. They were very different from the heirlooms of fine linen and soft furs that covered her four-poster bed in the Palace, but Jenna loved her old blankets and box bed as much as she ever did. She kneeled on the bed and peered out the tiny window for some minutes, looking up at the stars and down at the river far below, just as she had always done before she went to sleep. But the combination of the Dark of the Moon - which she sleepily remembered Aunt Zelda explaining to her one night on the Marram Marshes - with the thick, snowy clouds that covered most of the stars meant she could not see much at all. Her cupboard was colder than she remembered but before long Jenna too was asleep, curled up on the bed (which she had to be, because the bed was too short for her now), covered in the rough blankets, her fine fur-lined Princess cloak and her newly acquired Witch cloak. It was an odd combination but it kept her warm.

Septimus and Marcellus took turns through the night watching the door - two hours watching, two hours sleeping. When at about four in the morning the Darke Fog rolled down There and Back Again Row and pushed against the Big Red Door, Septimus was on watch. He woke Marcellus and together, on tenderhooks, they watched the door. The door tightened its hinges and long minutes passed, but the Darke Domaine did not get in.

The reason for this was not only Septimus's Magyk; it was also the Big Red Door itself. Benjamin Heap had suffused the Big Red Door with Magykal SafeScreens of his own before he gave it to his son, Silas. It was his way of ensuring that his son and grandchildren would be protected after he had gone. Benjamin's SafeScreens could not stop anything or anyone who had been invited in (like the midwife who had stolen Septimus) but they were pretty good at stopping anything that the Heaps had not invited over the threshold. Benjamin had never told Silas this, for he did not want his son to think that he doubted his Magykal powers - even though he did. But Sarah Heap had guessed long ago.

And so the Darke Domaine began its unrelenting onslaught - just as it was doing in the three other places in the Castle that had protected themselves: the Wizard Tower, the Hermetic Chamber - and Igor's own secret SafeChamber in Gothyk Grotto, which, in addition to Igor, contained Marissa, Matt and Marcus. But those behind the Big Red Door were safe for the moment. And when the light of the rising sun began to shine through the dusty mullioned window, Septimus and Marcellus relaxed their guard and fell asleep beside the glowing embers of the fire.

Sarah Heap woke with the sun as she always did. She stirred awkwardly, her neck stiff from the night spent on a threadbare rug with only a rocklike cushion for a pillow. She got up and walked stiffly over to the fire, stepping over Marcellus, and gently placing a pillow beneath Septimus's head. Then she added some logs to the embers and stood, arms wrapped around herself, watching the flames begin to wake. Silently she thanked Silas for all the stores he had laid in: logs neatly stacked under Jenna's bed, blankets, rugs and cushions, two cupboards full of jars of preserved fruit and vegetables, a whole box of dried WizStix, which would become strips of tasty dried fish or meat when reconstituted with the correct Spell (the tiny, sticklike Charm for which Silas had thoughtfully left tucked beside them). Plus, Silas had mended the loo. This had been the bane of Sarah's life when the Heap family had lived there. Plumbing was not one of the Ramblings' strong points and the lavatories - little more than huts perched precariously on the outside walls - were always messing up. But now, at long last, Silas had fixed it. All this, along with a late-night discovery of a WaterGnome hidden in the back of the cupboard, made Sarah think of Silas with wistful affection. She longed to thank him and apologize for all the times she had complained about him disappearing without saying where he was going. But most of all, she wished Silas knew that she was safe.

Sarah got out the WaterGnome and stood it on top of the cupboard where she had found it. She smiled; she could see why Silas had hidden it - it was one of the rude ones. But none the worse for that, Sarah thought, as the Gnome provided a stream of water for the kettle. Water was the thing she had been most worried about - hence the risky trip to the Well Hall. But now, thanks to Silas, they had a reliable supply.

Sarah hung the kettle over the fire and sat to watch it boil, remembering how she used to do this every morning. She had loved those rare moments to herself when all was quiet and peaceful. Of course when the children were very little she often had one or two of them sitting sleepily at her feet, but they were always quiet - and once they were older none of them ever woke up until she banged on the breakfast porridge pan. Sarah remembered how she would take the kettle off the fire just before it began to whistle, brew herself a cup of herb tea and sit quietly watching the sleeping forms strewn around the floor - just as she was doing now. Except, she thought wryly, as Thunder made his presence known in his own special way, she wouldn't have been staring at a fresh pile of horse poop.

Sarah got the shovel, opened the window and launched the steaming pile into the air. She leaned out and breathed in the sharp, fresh morning air, which was dusted with a scent of snow and river mud. Happy memories of MidWinter Feast days with Silas and the children came flooding back - along with a memory of one much less happy day fourteen years ago. She turned and looked at the sleeping form of her youngest son and thought that, whatever happened, he had now at last spent a night in the room he should have grown up in.

Sarah watched the pale, wintry sun edging up above the distant hills, shining weakly through the bare branches of the trees on the opposite side of the river. She sighed. It was good to see daylight once more - but who knew what the day would bring?

It brought another fight between Septimus and Simon.

Septimus and Marcellus had retreated to a quiet corner by Silas's bookshelves and were looking through his old Magyk books, searching for anything written about Darke Domaines. They found nothing of use. Most of Silas's books were common textbooks or cheap versions of more arcane books with pages missing - always the pages that promised something interesting.

Septimus, however, had just found a small pamphlet hidden inside an ink-spattered copy of Year III Magyk: Advanced Bothers when Simon wandered across to see if any of his old favorites were still on the shelves. He glanced down and saw the title of the pamphlet: The Darke Power of the Two-Faced Ring.

A dangerous and deeply flawed device, historically used by Darke Wizards and their acolytes, Septimus read.Traditionally worn on the left thumb. Once put on, the ring will travel in only one way and so cannot be removed except over the base of the thumb. The faces are thought to represent those of the two Wizards who created it. Each Wizard desired to possess the Ring and they fought to the death over it. (See this author's pamphlet on the formation of the Bottomless Whirlpool. Only six groats from Wywald's Witchery). After this the Ring passed from Wizard to Wizard, wreaking havoc. It is thought to have been instrumental in the Slime Plague at the Port, the horrific Night River Serpent attacks at the Ramblings and very possibly the Darke Pit over which the Municipal rubbish dump was eventually built. The Two-Faced Ring possesses Incremental Power - each wearer attains the Darke power of all the previous wearers. This power reaches its full potential only after it has been worn for thirteen lunar months. Although many say that the Two-Faced Ring is still in existence, the author does not believe this to be the case. It has not been heard of for many hundreds of years now, and the likelihood is that is has been irretrievably lost.

"Interesting," said Simon, reading over Septimus's shoulder. "But not entirely accurate."

Septimus's reply was short and to the point. "Go away," he said.

"Ahem." Marcellus coughed ineffectively.

"I am only trying to help," said Simon. "We all want to find a way to get rid of this Darke Domaine."

"We do," said Septimus, looking pointedly at Marcellus. "I'm not so sure about you."

Simon sighed, which annoyed Septimus. "Look, I don't do that stuff anymore. I really and truly don't."

"Ha!" said Septimus scornfully.

"Now, now, Apprentice. Remember what you promised your mother."

Septimus ignored Marcellus.

"You just don't get it, do you?" Simon sounded exasperated. "I made a mistake. Okay, it was a really bad mistake, but I am doing my best to put things right. I don't know what more I can do. And right now I could be really useful. I know more about this . . . stuff than both of you put together."

"I'll bet you do," snapped Septimus.

"Apprentice, I do think you should calm down and - "

Simon exploded. "You think just because you're Marcia's precious little Apprentice you know it all but you don't."

"Don't patronize me," said Septimus.

"Boys!" Suddenly Sarah was there. "Boys, what did I tell you?"

Septimus and Simon glared at each other. "Sorry, Mum," they both muttered between clenched teeth.

It was Marcellus who was the go-between. To a seething Septimus he said, "Apprentice, these are desperate times. And desperate times call for desperate measures. We need all the help we can get. And Simon has a great advantage; he knows the Darke and - "

"Too right," Septimus muttered under his breath.

Marcellus ignored the interruption. "And I do believe that he has changed. If anyone knows a way to defeat this Darke Domaine, it will be him and there is no need to make that kind of face, Septimus."

"Huh."

"We must do all that we can. Who knows how long we can keep the Darke Domaine out of the room? Who knows how long the poor people in the Castle can survive inside the Domaine? And indeed, who knows how long the Wizard Tower can hold out?"

"The Wizard Tower can hold out forever," said Septimus.

"Frankly, I doubt it. And what would be the point if it did? Soon it will be nothing more than an island marooned in a Castle of death."

"No!"

"Mark my words, Apprentice, the longer the Darke Domaine is in place, the more likely this is to be the case. Most people will survive for a few days. Others, perhaps those less lucky, will survive for longer but be driven mad by their experiences. We have a duty to do our utmost to prevent this. Do you not agree?"

Septimus nodded. "Yes," he said heavily.

Marcellus arrived where Septimus knew he'd been heading. "To this purpose I believe we should enlist the help of your brother."

Septimus could not bear the thought. "But we can't trust him," he protested.

"Apprentice, I truly believe we can trust him."

"No, we can't. He knowingly messes with the Darke. What kind of person does that?"

"People like us?" Marcellus said with a smile.

"That's different."

"And I believe your brother is different too."

"Too right."

"Apprentice, do not deliberately misunderstand me," Marcellus said sternly. "Your brother has made mistakes. He has paid - and indeed still is paying - a high price for them."

"And so he should."

"You are being a little vindictive, Apprentice. It is not an attractive quality in one with so much Magykal ability as you. You should be more magnanimous in your victory."

"My victory?"

"Ask yourself who anyone would rather be - Septimus Heap, ExtraOrdinary Apprentice, loved and respected by all in the Castle, with a brilliant future ahead of him, or Simon Heap, disgraced, exiled and living a hand-to-mouth existence in the Port with little to hope for?"

Septimus hadn't thought of it like that. He glanced over to Simon, who was alone, staring fixedly out the window. It was true; he wouldn't swap places with Simon for anything.

"Yeah," he said. "Yeah. Okay."

And so it was that, much to Sarah Heap's surprise and joy, her youngest and eldest sons spent the next few hours sitting together at the foot of Silas Heap's bookshelves, in deep discussion with Marcellus Pye - about whom Sarah had had a complete reversal of opinion. Occasionally one would take down a book from the shelves, but for the most part they sat quietly and apparently companionably together.

By the nightfall both Septimus and Marcellus Pye had learned a lot from Simon: How Simon had last seen the Two-Faced Ring on the slimy bones of his old Master, DomDaniel, as they were about to strangle him. How he had trapped the bones into a sack and thrown them into the Endless Cupboard in the Observatory. How Merrin must have somehow retrieved the ring from the slimy thumb bone of DomDaniel - the thought of which made them all shudder.

Septimus thought that if they got hold of Merrin and took the ring off him the Darke Domaine would disappear, but Simon had explained that once the Darke Domaine was in place it would take more than that to get rid of it - it would take the most powerful Magyk possible. When he mentioned the Paired Codes, Marcellus reluctantly recounted what had happened and a gloom fell.

"There is another way," said Simon after a while. "Apprentices to the same ExtraOrdinary Wizard share a Magykal link. Alther and Merrin were both Apprenticed to DomDaniel. And Alther is the most senior. There is a slim chance that he could UnDo the Darke Domaine, as it is the work of a more junior Apprentice. But . . ."

Septimus was listening with interest. "But what?" he asked. It was the first question he had asked Simon that was not an accusation too.

"But I am not sure if it works for ghosts," said Simon.

"It might though?"

"It might. It might not."

Septimus made up his mind. He would go to the Darke Halls and find Alther. It didn't matter whether Alther had the power Simon thought he had or not. Alther would know what to do, he was sure of that. He was their only hope.

"Marcellus," Septimus said. "You know how you said there were other Portals into the Darke Halls?"

"Yeess?" Marcellus knew what was coming.

"I want to find the most effective one. I shall go and bring Alther back."

Simon was horrified. "You can't go to the Darke Halls!"

"Yes, I can. I was going there anyway before all this happened."

Simon looked very concerned. "Septimus, be careful. That's why I wrote to you - apart from saying sorry for, um . . . trying to kill you. Which I am. I really am. You know that, don't you?"

"Yes, I think I do," said Septimus. "Thanks."

"Well, the last thing I want is for my little brother to get enmeshed with the Darke. It pulls you in. It changes you. It's a terrible thing. And the Darke Halls are the Darkest place of all."

"Simon, I don't want to go, but that's where Alther is," said Septimus. "And if there's a chance he can help then I want to take it. Anyway, I promised Alice I'd bring him back. And a promise is a promise."

Simon threw in his last card. "But what would Mum say?"

"Say about what?" Sarah - who had ears like a bat when it came to her children discussing her - called out from the other side of the room.

"Nothing, Mum," Simon and Septimus chorused in reply.

In the shadows of the bookshelves, Marcellus produced his pocket version of the almanac section of his book, I, Marcellus, and turned to the chapter headed Portal Calculations: Coordinates and Compass Points.

Night fell. Septimus Called yet again for Spit Fyre, although he now no longer expected his dragon to answer. The empty silence that followed his Call upset Septimus, but he tried not to let it show.

Sarah cooked up another stew, helped by Lucy, who wanted to know how to make a stew that was actually edible. After dinner Septimus, Simon and Marcellus returned to the bookshelves and, fortified by Sarah's stew, finished the first set of calculations, which showed where the Portal to the Darke Halls was - give or take half a mile. No one was very surprised at the result.

The evening drew on and a northeast wind began to blow up. It shook the windowpane and sent icy drafts into the room. The occupants wrapped themselves in blankets and settled down for the night. Soon the room behind the Big Red Door fell quiet.

Shortly after midnight, on the other side of the Big Red Door, a Thing arrived. It regarded the door with interest. It placed its ragged hands on the shiny red wood and winced as they touched the Camouflaged Magyk that covered the surface. Unnoticed by Marcellus - who was meant to be keeping watch but had actually dozed off - the door shuddered slightly and tightened its hinges.

The Thing sloped off down the corridor, muttering Darkely to itself.