In the fourth year after the Grey Tide, water can kill, medicine can trigger mutations, and survival often comes down to a gamble.
But quarantine inspector Gu Yan can see a different picture--
A dying dog has three possible evolutionary paths;
A pool of poisonous water conceals moss that can consume the toxin;
A thug impervious to blades and bullets has all his strength tied to a few materials that could break at any moment.
He can see the way forward, but he can't spare anyone the price they must pay.
When the Black Sails, who control the water treatment plant, demand human lives in exchange for water, Gu Yan leads forty-three survivors to establish a White Tower at an abandoned animal quarantine station: they make their own water, guard their own gates, and let everyone who undergoes evolutionary intervention choose for themselves.
But more and more people begin waiting for him to provide the "right answer."
And deep in the city, someone has already decided that humanity needs only one correct evolutionary path.
