In a world ravaged by the Root, a destructive, plant-like species that spreads like a virus, a lone survivor known as the Traveler is rescued and brought to the last-refuge settlement of Ward 13. There they discover the World Stone, a crystal that opens portals to other realms, and are drawn into a desperate mission after companions vanish through it. Journeying across strange and hostile worlds, from Root-choked forests to a planet-sized starship, the Traveler searches for the missing, allies with a guardian known as the Keeper, and races to stop the Root before it consumes everything.