At the turn of the twenty-second century, scientists make a breakthrough in human spaceflight. Through a revolutionary method known as somaforming, astronauts can survive in hostile environments off Earth using synthetic biological supplementations. They can produce antifreeze in subzero temperatures, absorb radiation and convert it for food, and conveniently adjust to the pull of different gravitational forces. With the fragility of the body no longer a limiting factor, human beings are at last able to journey to neighboring exoplanets long known to harbor life.

A team of these explorers, Ariadne O'Neill and her three crewmates, are hard at work in a planetary system fifteen light-years from Sol, on a mission to ecologically survey four habitable worlds. But as Ariadne shifts through both form and time, the culture back on Earth has also been transformed. Faced with the possibility of returning to a planet that has forgotten those who have left, Ariadne begins to chronicle the story of the wonders and dangers of her mission, in the hope that someone back home might still be listening.