In a world where the cells that make up our bodies are not committed to any one organism, Marla is confronted by the fickleness of her cytes, and resolves to understand them with help from Ada, a centuries-old Flourisher. Swappers like Ruth embrace fluidity, and meet with others to exchange cytes, seeking the perfect mix. But Ruth faces her own crisis, and as the technology to manipulate cytes advances, all three are drawn into a struggle to shape the future of life.