The sun rose slowly, as if it wasn't sure it was worth all the effort.

Another Disc day dawned, but very gradually, and this is why.

When light encounters a strong magical field it loses ail sense of urgency. It slows right down. And on the Discworld the magic was embarrassingly strong, which meant that the soft yellow light of dawn flowed over the sleeping landscape like the caress of a gentle lover or, as some would have it, like golden syrup. It paused to fill up valleys. It piled up against mountain ranges. When it reached Cori Celesti, the ten mile spire of grey stone and green ice that marked the hub of the Disc and was the home of its gods, it built up in heaps until it finally crashed in great lazy tsunami as silent as velvet, across the dark landscape beyond.

It was a sight to be seen on no other world.

Of course, no other world was carried through the starry infinity on the backs of four giant elephants, who A'ere themselves perched on the shell of a giant turtle. His name – or Her name, according to another school of thought – was Great A'Tuin; he – or, as it might be, she – will not take a central role in what follows but it is vital to an understanding of the Disc that he – or she – is there, down below the mines and sea ooze and fake fossil bones put there by a Creator with nothing better to do than upset archeologists and give them silly ideas.

Great A'Tuin the star turtle, shell frosted with frozen methane, pitted with meteor craters, and scoured with asteroidal dust. Great A'Tuin, with eyes like ancient seas and a brain the size of a continent through which thoughts moved like little glittering glaciers. Great A'Tuin of the great slow sad flippers and star-polished carapace, labouring through the galactic night under the weight of the Disc. As large as worlds. As old as Time. As patient as a brick.

Actually, the philosophers have got it all wrong. Great A'Tuin is in fact having a great time.

Great A'Tuin is the only creature in the entire universe that knows exactly where it is going.

Of course, philosophers have debated for years about where Great A'Tuin might be going, and have often said how worried they are that they might never find out.

They're due to find out in about two months. And then they're really going to worry . . .

Something else that has long worried the more imaginative philosophers on the Disc is the question of Great A'Tuin's sex, and quite a lot of time and trouble has been spent in trying to establish it once and for all.

In fact, as the great dark shape drifts past like an endless tortoiseshell hairbrush, the results of the latest effort are just coming into view.

Tumbling past, totally out of control, is the bronze shell of the Potent Voyager, a sort of neolithic spaceship built and pushed over the edge by the astronomer-priests of Krull, which is conveniently situated on the very rim of the world and proves, whatever people say, that there is such a thing as a free launch.

Inside the ship is Twoflower, the Disc's first tourist. He had recently spent some months exploring it and is now rapidly leaving it for reasons that are rather complicated but have to do with an attempt to escape from Krull.

This attempt has been one thousand per cent successful.

But despite all the evidence that he may be the Disc's last tourist as well, he is enjoying the view.

Plunging along some two miles above him is Rincewind the wizard, in what on the Disc passes for a spacesuit. Picture it as a diving suit designed by men who have never seen the sea. Six months ago he was a perfectly ordinary failed wizard. Then he met Twoflower, was employed at an outrageous salary as his guide, and has spent most of the intervening time being shot at, terrorised, chased and hanging from high places with no hope of salvation or, as is now the case, dropping from high places.

He isn't looking at the view because his past life keeps flashing in front of his eyes and getting in the way. He is learning why it is that when you put on a spacesuit it is vitally important not to forget the helmet.

A lot more could be included now to explain why these two are dropping out of the world, and why Twoflower's Luggage, last seen desperately trying to follow him on hundreds of little legs, is no ordinary suitcase, but such questions take time and could be more trouble than they are worth. For example, it is said that someone at a party once asked the famous philosopher Ly Tin Weedle 'Why are you here?' and the reply took three years.

What is far more important is an event happening way overhead, far above A'Tuin, the elephants and the rapidly-expiring wizard. The very fabric of time and space is about to be put through the wringer.

The air was greasy with the distinctive feel of magic, and acrid with the smoke of candles made of a black wax whose precise origin a wise man wouldn't inquire about.

There was something very strange about this room deep in the cellars of Unseen University, the Disc's premier college of magic. For one thing it seemed to have too many dimensions, not exactly visible, just hovering out of eyeshot. The walls were covered with occult symbols, and most of the floor was taken up by the Eightfold Seal of Stasis, generally agreed in magical circles to have all the stopping power of a well-aimed half brick.

The only furnishing in the room was a lectern dark wood, carved into the shape of a bird – well, to be frank, into the shape of a winged thing it is probably best not to examine too closely – and on the lectern, fastened to it by a heavy chain covered in padlocks, was a book.

A large, but not particularly impressive, book. Other books in the University's libraries had covers inlaid with rare jewels and fascinating wood, or bound with dragon skin. This one was just a rather tatty leather. It looked the sort of book described in library catalogues as 'slightly foxed', although it would be more honest to admit that it looked as though it had been badgered, wolved and possibly beared as well.

Metal clasps held it shut. They weren't decorated, they were just very heavy – like the chain, which didn't so much attach the book to the lectern as tether it.

They looked like the work of someone who had a pretty definite aim in mind, and who had spent most of his life making training harness for elephants.

The air thickened and swirled. The pages of the book began to crinkle in a quite horrible, deliberate way, and blue light spilled out from between them. The silence of the room crowded in like a fist, slowly being clenched.

Half a dozen wizards in their nightshirts were taking turns to peer in through the little grille in the door. No wizard could sleep with this sort of thing going on – the build-up of raw magic was rising through the university like a tide.

'Right,' said a voice. What's going on? And why wasn't I summoned?'

Galder Weatherwax, Supreme Grand Conjuror of the Order of the Silver Star, Lord Imperial of the Sacred Staff, Eighth Level Ipsissimus and 304th Chancellor of Unseen University, wasn't simply an impressive sight even in his red nightshirt with the hand-embroidered mystic runes, even in his long cap with the bobble on, even with the Wee Willie Winkie candlestick in his hand. He even managed to very nearly pull it off in fluffy pompom slippers as well.

Six frightened faces turned towards him.

'Um, you were summoned, lord,' said one of the under-wizards.

'That's why you're here,' he added helpfully.

'I mean why wasn't I summoned before?' snapped Galder, pushing his way to the grille.

'Um, before who, lord?' said the wizard.

Galder glared at him, and ventured a quick glance through the grille.

The air in the room was now sparkling with tiny flashes as dust motes incinerated in the flow of raw magic. The Seal of Stasis was beginning to blister and curl up at the edges.

The book in question was called the Octavo and, quite obviously, it was no ordinary book.

There are of course many famous books of magic. Some may talk of the Necrotelicomnicon, with its pages made of ancient lizard skin; some may point to the Book of Going Forth Around Elevenish, written by a mysterious and rather lazy Llamaic sect; some may recall that the Bumper Fun Grimoire reputedly contains the one original joke left in the universe. But they are all mere pamphlets when compared with the Octavo, which the Creator of the Universe reputedly left behind – with characteristic absent-mindedness – shortly after completing his major work.

The eight spells imprisoned in its pages led a secret and complex life of their own, and it was generally believed that —

Galder's brow furrowed as he stared into the troubled room. Of course, there were only seven spells now. Some young idiot of a student wizard had stolen a look at the book one day and one of the spells had escaped and lodged in his mind. No-one had ever managed to get to the bottom of how it had happened. What was his name, now? Winswand?

Octarine and purple sparks glittered on the spine of the book. A thin curl of smoke was beginning to rise from the lectern, and the heavy metal clasps that held the book shut were definitely beginning to look strained.

'Why are the spells so restless?' said one of the younger wizards.

Galder shrugged. He couldn't show it, of course, but he was beginning to be really worried. As a skilled eighth-level wizard he could see the half-imaginary shapes that appeared momentarily in the vibrating air, wheedling arid beckoning. In much the same way that gnats appear before a thunderstorm, really heavy build-ups of magic always attracted things from the chaotic Dungeon Dimensions – nasty Things, all misplaced organs and spittle, forever searching for any gap through which they might sidle into the world of men. [1]

This had to be stopped.

'I shall need a volunteer,' he said firmly.

There was a sudden silence. The only sound came from behind the door. It was the nasty little noise of metal parting under stress.

'Very well, then,' he said. 'In that case I shall need some silver tweezers, about two pints of cat's blood, a small whip and a chair —'

It is said that the opposite of noise is silence. This isn't true. Silence is only the absence of noise. Silence would have been a terrible din compared to the sudden soft implosion of noiselessness that hit the wizards with the force of an exploding dandelion clock.

A thick column of spitting light sprang up from the book, hit the ceiling in a splash of flame, and disappeared.

Galder stared up at the hole, ignoring the smouldering patches in his beard. He pointed dramatically.

To the upper cellars!' he cried, and bounded up the stone stairs. Slippers flapping and nightshirts billowing he other wizards followed him, falling over one another in their eagerness to be last.

Nevertheless, they were all in time to see the fireball of occult potentiality disappear into the ceiling of the room above.

'Urgh,' said the youngest wizard, and pointed to the floor.

The room had been part of the library until the magic had drifted through, violently reassembling the possibility particles of everything in its path. So it was reasonable to assume that the small purple newts had been part of the floor and the pineapple custard may once have been some books. And several of the wizards later swore that the small sad orang outang sitting in the middle of it all looked very much like the head librarian.

Galder stared upwards. 'To the kitchen!' he bellowed, wading through the custard to the next flight of stairs.

No-one ever found out what the great cast-iron cooking range had been turned into, because it had broken down a wall and made good its escape before the dishevelled party of wild-eyed mages burst into the room. The vegetable chef was found much later hiding in the soup cauldron, gibbering unhelpful things like The knuckles! The horrible knuckles!'