The old world is buried. A new one has been forged atop the shifting dunes. Here in this land of howling wind and infernal sand, four siblings find themselves scattered and lost. Their father was a sand diver, one of the elite few who could travel deep beneath the desert floor and bring up the relics and scraps that keep their people alive. But their father is gone. And the world he left behind might be next.

The world outside has grown toxic, the view of it limited, talk of it forbidden. The remnants of humanity live underground in a single silo. But there are always those who hope, who dream. These are the dangerous people, the residents who infect others with their optimism. Their punishment is simple. They are given the very thing they want: They are allowed to go outside.

For centuries, men and women have manned lighthouses to ensure the safe passage of ships. It is a lonely job and a thankless one for the most part. Until something goes wrong. Until a ship is in distress. In the twenty-third century, this job has moved into outer space. A network of beacons allows ships to travel across the Milky Way at many times the speed of light. These beacons are built to be robust. They never break down. They never fail. At least, they aren’t supposed to.

Founded by the mysterious genius known as the Designer, the archipelago of Prospera lies hidden from the horrors of a deteriorating outside world. In this island paradise, Prospera’s lucky citizens enjoy long, fulfilling lives until the monitors embedded in their forearms, meant to measure their physical health and psychological well-being, fall below 10 percent. Then they retire themselves, embarking on a ferry ride to the island known as the Nursery, where their failing bodies are renewed, their memories are wiped clean, and they are readied to restart life afresh.

Shana wakes up one morning to discover her little sister in the grip of a strange malady. She appears to be sleepwalking. She cannot talk and cannot be woken up. And she is heading with inexorable determination to a destination that only she knows. But Shana and her sister are not alone. Soon, they are joined by a flock of sleepwalkers from across America, on the same mysterious journey. And like Shana, there are other "shepherds" who follow the flock to protect their friends and family on the long dark road ahead.