Septimus sat next to the pile of bones feeling bad. Really bad. Really, really bad. He thought of Beetle, Sealed into the Hermetic Chamber, and himself marooned in the Darke Halls and he knew that there was no hope left for either of them.

He stretched out his hands and looked at his Dragon Ring, the only thing he had left for company. He saw the warm yellow glow and the green emerald eye and he thought it was true; it was a pretty ring. And suddenly something clicked - he understood the little ghost's chattering about the ring. He wore his Dragon Ring on his right hand - he knew he did. He could even feel it on his right hand, on the index finger, where it always was. And yet, when he looked at his hands, the ring appeared to be on his left index finger. Septimus stared at his hands, uncomprehending. And then he understood. That was it. The ghost been giving him a clue - in the Darke Halls everything was Reversed, so when he had thought he was taking the left turning, he had in fact been taking the right. So maybe Simon had not deceived him after all. Maybe . . .

Septimus leaped to his feet and, with renewed hope, he set off once more. He took the apparent right-hand entrance of the first three and found himself in yet another great Hall. He speeded up, almost running in his wish to discover if this really was the secret to finding his way to Dungeon Number One. After choosing an apparent right-hand passageway leading from a small archway that very soon pided into two flights of steps - of which he took the right-hand flight - he pushed open a heavy door and found himself in a huge cavern that was actually lit. Great torches flared from niches carved into the smooth rock walls, illuminating the soaring heights of the Hall, casting long shadows across the smooth rock floor. Septimus felt like yelling with joy. He was getting somewhere now, he knew he was.

As he jogged along he began to encounter Things, Magogs, Wizards, Witches and all manner of misshapen creatures - and he was glad to see every single one. Each and every one passed him by and paid him no attention. His Darke Disguise still did what it was meant to do - it presented Septimus as something Darke, something that was one of them.

Septimus reckoned he must now be walking beneath the Castle. He began to pass by archways protected by metal grills, which he suspected led into secret entrances somewhere in the Castle - entrances that even Marcia did not know about. There was a buzz of excitement in the air, which Septimus guessed was to do with the Darke events far above in the Castle itself. He passed by two Wizards who had left the Wizard Tower in disgrace a few years ago and heard one say excitedly, "Our time has come."

And then, at last, he saw ahead of him a portico. Gold streaks in the lapis lazuli of its pillars glistened in the light of the torches and Septimus knew that this was the one that would take him into the antechamber to Dungeon Number One. Some minutes later, feeling so excited that he could hardly breathe, Septimus reached the portico.

As he went to step through, Tertius Fume - self-appointed busybody who terrified many of the ghosts - accosted him with a touch so cold that it felt burning hot. Septimus stopped, his heart beating fast. This put the Darke Disguise to its greatest test so far. Surely Tertius Fume would recognize him?

It appeared the ghost did not. He glared at Septimus with his piercing, goatlike eyes and demanded, "Who be you?"

Septimus was ready. "Sum."

"How be you?"

"Darke."

"What be you?"

"The Apprentice of the Apprentice of the Apprentice of DomDaniel."

Tertius Fume looked surprised. He stopped his questioning and tried to figure out who exactly Septimus was. Septimus took advantage of the ghost's confusion and stepped through the entrance. He was probably the first person to feel utter delight at finding himself in the large, round chamber lined with black bricks, stuffed full of depressed ghosts. Now all he had to do was to find one ghost in particular.

Septimus scanned the room and his heart leaped. There was Alther, sitting motionless on a stone bench set into the wall, his eyes closed.

Tertius Fume had given up trying to figure out who Septimus was - there were too many possibilities. The ghost followed him into the antechamber.

"Why come you here?" he demanded.

Septimus ignored Tertius Fume and began to make his way over to Alther. Tertius Fume followed like a storm cloud as Septimus dodged from side to side to avoid Passing Through the throng of ghosts. Eventually, with a feeling of elation, Septimus reached Alther's side. He had imagined this moment many times as he had traveled through the Darke Halls. He had longed to see Alther's expression as the ghost looked up and Saw through his Darke Disguise to the person he really was. But to his disappointment, nothing happened - Alther did not react. He seemed oblivious to his surroundings. His eyes remained closed and he sat still as a statue. Septimus knew that Alther had gone somewhere deep within himself.

Mindful of Marcellus's instructions to speak only the set responses in the presence of the Darke - and with Tertius Fume hovering at his shoulder, he was certainly in that - Septimus stood wondering how to reach Alther. Tertius Fume solved his problem.

"Why come you here?" he demanded once again.

Loudly, hoping that Alther would recognize his voice, Septimus said, "I seek the Apprentice of DomDaniel."

The moment that Alther recognized him was one of the best moments in Septimus's life. Alther's eyes opened slowly and Septimus saw recognition dawn. But Alther did not move an inch. His glance flicked sideways, took in Tertius Fume, and closed again. Septimus was elated. Alther understood. Alther was with him once again.

Tertius Fume did not notice Alther's awakening, as he was too busy scrutinizing the newcomer. There was, he was sure, something odd about Sum - but what it was, he could not tell. The ghost gave Septimus a goaty gloat of a smile and replied, "Then, Sum, you are in the wrong place. The Apprentice of DomDaniel is doing well - surprisingly well, I hear - above."

Septimus bowed and smiled in reply.

Tertius Fume mockingly returned the bow and drifted away.

Septimus sat down beside Alther. He knew Tertius Fume was suspicious and he had to work fast. He got straight to the point. "Marcia has given me the Revoke for the Banish. I have come to deliver it." He glanced at the ghost. To any onlooker, Alther looked the same. He was sitting stone still with his eyes closed. But Septimus could tell that the ghost was poised like a cat waiting to pounce. He was ready to go.

Septimus took a deep breath and in a low monotone, he began the Revoke. He longed to rush through the words and get it over with before Tertius Fume noticed what was happening, but he knew he could not. The Revoke must mirror the original form of the Banish. It must last, to the microsecond, the same amount of time. It must begin at the end of the Banish and end at the beginning.

Five and a half seconds before the end of the Revoke, Tertius Fume finally put two and two together. From a shortlist of seven, he had worked out who Septimus was. He was across the antechamber in a flash, Passing Through any ghost that got in his way. If it hadn't been for a particularly grumpy ghost - an unlucky bricklayer who had fallen into Dungeon Number One while repairing the wall - Tertius Fume would have been at Septimus's side in time to disrupt the Revoke. But thanks to the bricklayer, he arrived at the very moment the last words - "Overstrand Marcia I" - were being spoken.

Like a coiled spring, Alther leaped to his feet. In a most unghostly fashion, he grabbed Septimus by the hand and headed for the Darke vortex that spun in the very center of the antechamber. Tertius Fume raced after them but he was too late. Septimus and Alther were sucked into the vortex, but the still-Banished Tertius Fume was thrown clear and sent spinning across the antechamber like any new ghost hurled from Dungeon Number One.

Septimus and Alther were free. Together they crashed up through the layers of bones and despair, burst out through the sludge and slime, and hurtled into the chimney of Dungeon Number One. Septimus was propeled upward with the force. High above him he saw the iron rungs of the ladder that he must reach. Up, up he went, but just as he was within an arm's length of the lowest rung he felt his momentum fade and Septimus knew that he would not reach it. Soon he would drop back into the mire at the bottom of the dungeon - the mire from which few escaped. Dismayed, Alther saw gravity begin to take its hold on Septimus.

"Flyte, Septimus! Think Flyte!" the ghost urged, hovering beside Septimus. "Think it, be it, do it. Flyte!"

And so, remembering a time on the edge of an icy cliff beside an abyss, Septimus thought of his ancient Flyte Charm - now languishing in the bottom of a pot in the Manuscriptorium Vaults - and he felt gravity loose its hold and allow the momentum to continue. The next moment his hand had clutched the icy iron rung at the foot of the ladder and Septimus knew he was safe.

Alther kept pace with Septimus as he climbed the rungs. Far below the howl of the vortex grew ever fainter as he struggled upward and now, at last, he could see the thick iron door at the top, streaked with rust. On the very top rung Septimus halted and, clinging on with one hand, he fumbled in his buttoned pocket for the precious key. It took him many long, tired minutes to undo the buttons, but finally he took out the key, looped its cord around his wrist for safety, pushed it into the lock and turned it.

The door swung open and the Darke Fog tumbled in fast, taking Septimus by surprise and knocking him backward. He would have fallen had not two pairs of strong arms grabbed him and dragged him out of the door like a sack of potatoes.

"Sep! You're safe! And Uncle Alther! Oh, you're both safe!" Jenna's voice was distant in the Darke Fog but there was no mistaking the laughter and relief in it.

Septimus sat propped up against the little brick cone of the top of Dungeon Number One, too tired to do anything but smile. Jenna and Nicko, both swathed in the voluminous witch's cloak, regarded him with answering smiles. There was nothing anyone needed to say - they were all together again.

But Alther had something to say. "Hmm," he murmured. "You've let the old place get into a bit of a state while I was away."