In the kingdom of Krull on the edge of the Circle Sea, the Arch-astronomer watches the slaves polish the bronze sides of a spaceship shaped like a giant metal fish. In three days, the Disc will be in a perfect position to observe the tail of Great A’Tuin. The ship will be lowered over the edge of the Disc, and the astrozoologists of the Discworld will be able to determine the sex of the great star turtle. Everything is ready for launch. All they need is a couple of human sacrifices, and the god Fate, still annoyed over having lost the game to the Lady in Part 2, has demanded Rincewind and Twoflower.

Rincewind and Twoflower are adrift on the Circle Sea, not far from Krull. Six months have passed since the burning of Ankh-Morpork. After adventures all over the Disc, they have escaped a slave ship but were forced to leave the Luggage behind. They now find themselves in a leaky boat, drifting toward the Rimfall. Their boat is surrounded by flotsam. Rincewind sees a little frog struggling against the current and, in a rare moment of sympathy, rescues it. They are about to go over the Rim when the boat comes up against the Circumfence that runs around the edge of the world and prevents things from falling over.