A powerful pathogen is killing every inhabitant of Earth. Amid the chaos, an experimental spaceship designed as an ark is prepared for departure. After it transports Maria Ramos and six other people to a mysterious planet, they learn that their minds have been uploaded as digital consciousnesses and that all other passengers died during the journey. It's up to these seven minds to save the human race. The ark contains frozen human tissues that can be used to grow colonists. But after the ship's AI transfers the surviving consciousnesses into robots, some of them plot to steal and inhabit some colonist bodies and destroy the rest, even if it means that humanity dies out. After Maria's murder, a community of mixed human and native blood develops, but it faces its own extinction event. Clues for survival come from a planet-wide AI created by an ancient civilization, astounding messages from a distant source, and an intriguing traveler who seems to know much about Maria and her long-ago efforts to promote sentient life. Castle of Sand is a compelling and inspirational exploration of what it means to be human.