In a world where going Gray means certain death, Scout's relentless quest for a cure becomes a race against time to save her loved ones. When she's nearly claimed by a deadly blizzard, her survival is miraculously ensured by the unexpected visitation of a long-lost friend. Shortly thereafter, she and her companions uncover the life-saving cure, but their triumph is short-lived as Grays overrun their sanctuary.

Once again on the run, with nowhere to turn, Scout and her companions find the Hill, the last bastion of hope in this world ravaged by global devastation. But, when an army of Grays led by an old adversary breaches the Hill's defenses, their illusion of paradise is shattered.

In this final, desperate fight against overwhelming odds, the fate of humanity teeters on the brink. If Scout and her friends falter, the priceless cure will be lost, and mankind is doomed.