From the introduction:
On the very edge of night a handful of outrigger canoes ride the dawn-winds, chasing the dark ice-islands on which they hope their loved ones still survive.
Imagine a world like the Pacific. The endless sea is dotted by semi-tropical islands - small, lush paradises populated by isolated societies, each slightly different from the next. Fishers and farmers, they are proud people, mostly peaceful, sometimes warlike. There the days are long - very long, weeks by our standards, the dawn coming no faster than a swift-sailing outrigger canoe - and the nights, longer still. And now, in the night has come a horror, unseen or unheard of before, a horror which has ripped the tribe to shreds - huge islands of ice, swarming with the dead. From the ice the dead creatures descend on the islands and carry away the living, carry them away into the endless night in which the ice-islands sail.