It is a Darke and stormy night.

Black clouds hang low over the Castle, shrouding the golden pyramid at the top of the Wizard Tower in a dim mist. In the houses far below, people stir uneasily in their sleep as the rumble of thunder enters their dreams and sends nightmares tumbling from the sky.

Like a giant lightning conductor, the Wizard Tower rears high above the Castle rooftops, Magykal purple and indigo lights playing around its iridescent silver sheen. Inside the Tower the duty Storm Wizard prowls the dimly lit Great Hall, checking the StormScreen and keeping an eye on the UnStable window, which has a tendency to panic in a storm. The duty Storm Wizard is a little on edge. Magyk is not usually affected by a storm, but all Wizards know about the Great Lightning Strike of Long Ago, which briefly drained the Wizard Tower of its Magyk and left the rooms of the ExtraOrdinary Wizard badly scorched. No one wants that to happen again - particularly the duty Storm Wizard.

At the top of the Wizard Tower in her as yet unscorched four-poster bed, Marcia Overstrand groans as a familiar nightmare flickers through her sleep. A loud craaaack of lightning splits open the cloud above the Tower and zips harmlessly to earth down the duty Storm Wizard's hastily conjured Conductor. Marcia sits bolt upright, dark curly hair awry, trapped in her nightmare. Suddenly her green eyes open wide with surprise as a purple ghost shoots through the wall and skids to a halt beside the bed.

"Alther!" gasps Marcia. "What are you doing?"

The tall ghost with long white hair tied back in a ponytail is wearing bloodstained ExtraOrdinary Wizard robes. He looks flustered.

"I really hate it when that happens," he gasps. "Got Passed Through. By lightning."

"I'm very sorry, Alther," Marcia replies grumpily, "but I don't see why you had to come and wake me up just to tell me that. You may not need to sleep anymore, but I certainly do. Anyway, it serves you right for being out in a storm. Can't think why you want to do that - argh!"

Another craaaack of lightning illuminates the purple glass of Marcia's bedroom window and makes Alther appear almost transparent.

"I wasn't out there for the fun of it, Marcia, believe me," says Alther, equally grumpily. "I was coming to see you. As you requested."

"As I requested?" says Marcia blearily. She is still half in her nightmare about Dungeon Number One - a nightmare that always comes when a storm is playing around the top of the Wizard Tower.

"You requested - ordered would be a better way of putting it - that I track down Tertius Fume and tell you when I had found him," says Alther.

Marcia is suddenly wide-awake. "Ah," she says.

"Ah, indeed, Marcia."

"So you have found him?"

The ghost looks pleased with himself. "Yup," he says.

"Where?"

"Where do you think?"

Marcia throws back the bedcovers, slips out of bed and pulls on her thick woolen gown - it is cold at the top of the Wizard Tower when the wind blows. "Oh, for goodness' sake, Alther," she snaps as she pushes her feet into the purple rabbit slippers that Septimus gave her for her birthday. "I wouldn't ask if I knew, would I?"

"He's in Dungeon Number One," Alther says quietly.

Marcia sits down on the bed rather suddenly. "Oh," she says, her nightmare replaying itself at double speed. "Bother."

Ten minutes later, two purple-clothed figures can be seen scurrying along Wizard Way. They are both trying to keep out of the needle-sharp rain that sweeps up the Way, Passing Through the leading figure and soaking the one close behind. Suddenly the first figure pes down a small alleyway, closely followed by the second. The alleyway is dark and smelly but at least it is sheltered from the near-horizontal rain.

"Are you sure it's down here?" asks Marcia, glancing behind. She doesn't like alleyways.

Alther slows his pace and drops back to walk beside Marcia. "You forget," he says with a smile, "that not so very long ago, I came down here quite often."

Marcia shudders. She knows that it was Alther's faithful visits that kept her alive in Dungeon Number One.

Alther has stopped beside a blackened, brick-built cone that looks like one of the many disused Lock-Ups that can still be seen scattered around the Castle. Somewhat unwillingly, Marcia joins the ghost; her mouth is dry and she feels sick. This is where her nightmare always begins.

Lost in her thoughts, Marcia waits for Alther to unlock the small iron door, which is pockmarked with rust. The ghost gives her a quizzical look. "No can do, Marcia," he says.

"Huh?"

"Wish I could," says Alther wistfully, "but, unfortunately, you are going to have to open the door."

Marcia comes to her senses. "Sorry, Alther." She takes out the Universal Castle Key from her ExtraOrdinary Wizard belt. Only three of these keys were ever made, and Marcia has two of them: one of her own in her capacity as ExtraOrdinary Wizard, and one that she is keeping safe for Jenna Heap until the day she becomes Queen. The third is lost.

Making an effort to steady her hand, Marcia pushes the iron key into the lock and turns it. The door swings open with a creak that at once takes her back to a terrifying snowy night when a phalanx of guards threw her through the door and sent her tumbling into the darkness.

A foul smell of rotting meat and burned pumpkin tumbles out into the alleyway, and a trio of curious local cats screech and head for home. Marcia wishes she could do the same. Nervously she fingers the lapis lazuli amulet - the symbol and source of her power as ExtraOrdinary Wizard - that she wears around her neck and, to her relief, it is still there - unlike the last time she passed through the door.

Marcia's courage returns. "Right, Alther," she says. "Let's get him."

Alther grins, relieved to see Marcia back in form. "Follow me," he says.

Dungeon Number One is a deep, dark chimney with a long ladder attached to the inside of the top half. The bottom half is ladder-free, lined with a thick layer of bones and slime. Alther's purple floating form drifts down the ladder but Marcia steps carefully - very carefully - down each rung, chanting an UnHarm Spell under her breath, with a Begird and Preserve in readiness for both her and Alther - for even ghosts are not immune to the Darke Vortices that swirl around the base of Dungeon Number One.

Slowly, slowly, the figures descend into the thick gloom and stench of the dungeon. They are going much farther down than Marcia expected. Alther had assured her that their quarry was "only lurking around the top, Marcia. Nothing to worry about."

But Marcia is worried. She begins to fear a trap. "Where is he?" she hisses.

A deep, hollow laugh answers her question, and Marcia very nearly lets go of the ladder.

"There he is!" says Alther. "Look, down there." He points into the narrow depths and, far below, Marcia sees the goatlike face of Tertius Fume leering up at them, an eerie green glowing in the darkness. "You can see him, you can do the Banish from here," says Alther, lapsing into tutor mode with his ex-pupil. "The chimney will concentrate it."

"I know," says Marcia tetchily. "Please be quiet, Alther." She begins to chant the words that all ghosts dread - the words that will Banish them to the Darke Halls forever.

"I, Marcia Overstrand . . ."

The greenish figure of Tertius Fume begins to rise up the chimney toward them. "I am warning you, Marcia Overstrand - stop that Banish now." His harsh voice echoes around them.

Tertius Fume gives Marcia the creeps, but she is not deflected. She carries on with the chant, which must last for precisely one minute and be completed without hesitation, repetition or deviation. Marcia knows that the slightest falter means she must begin again.

Tertius Fume knows this too. He continues his approach, walking up the side of the wall like a spider, hurling insults, counter chants and bizarre fragments of songs at Marcia to try to put her off.

But Marcia will not be deflected. Doggedly she continues, blanking out the ghost. But as she embarks upon the closing lines of the Banish - "your time above this earth is done, you'll see no more the sky, the sun" - out of the corner of her eye, Marcia sees the ghost of Tertius Fume drawing ever closer. A stab of worry shoots through her - what is he doing? Marcia reaches the very last line. The ghost is inches away from her and Alther. He looks up, excited - almost exultant.

Marcia ends the chant with the dreaded words, "By the power of Magyk, to the Darke Halls, I you . . ."

As Marcia reaches the very last word, Tertius Fume stretches his hand up to Alther and Merges with his big toe. Alther recoils from the touch but is too late.

"Banish!"

Suddenly Marcia is alone in the chimney of Dungeon Number One. Her nightmare has come true. "Alther!" she screams. "Alther, where are you?"

There is no reply. Alther is Banished.