It was a dark, blustery afternoon in spring, and the city of London was chasing a small mining town across the dried-out bed of the old North Sea.

In happier times, London would never have bothered with such feeble prey. The great Traction City had once spent its days hunting far bigger towns than this, ranging north as far as the edges of the Ice Waste and south to the shores of the Mediterranean. But lately prey of any kind had started to grow scarce, and some of the larger cities had begun to look hungrily at London. For ten years now it had been hiding from them, skulking in a damp, mountainous, western district which the Guild of Historians said had once been the island of Britain. For ten years it had eaten nothing but tiny farming towns and static settlements in those wet hills. Now, at least, the Lord Mayor had decided that the time was right to take his city back over the land-bridge into the Great Hunting Ground.

It was barely halfway across when the look-outs on the high watch-towers spied the mining town, gnawing at the salt-flats twenty miles ahead. to the people of London it seemed like a sign from the gods, and even the Lord Mayor (who didn't believe in gods or signs) thought it was a good beginning to the journey east, and issued the order to give chase.

The mining town saw the danger and turned tail, but already the huge caterpillar tracks under London were starting to roll faster and faster. Soon the city was lumbering in pursuit, a moving mountain of metal which rose in seven tiers like the layers of a wedding cake, the lower levels wreathed in engine-smoke, the villas of the rich gleaming white on the higher decks, and above it all the cross on top of St Paul's Cathedral glinting gold, two thousand feet above the ruined earth.