Created in response to the collapse of the Earth's biosphere, the great domed city of Fairhaven was designed as a self-sustaining human habitat, managed by a sophisticated artificial intelligence. But after centuries of gradual decline, Fairhaven can no longer support more than a tiny fraction of its once-burgeoning populace. Now, deadly robotic predators known as fells stalk the city's residential zone, and the people live in shanty towns, out beyond the encircling river. However, finished goods are still delivered from the city's automated factories to the empty homes, and so harvesters - teenagers, and children as young as age ten - cross the river every morning to search for food and other essentials. The fells choose their targets by age, and so the youngest harvesters are usually safe from attack; but as they grow older, their work becomes ever more dangerous. Most will never see their twentieth birthday.