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.