In 1348 Aquitaine, during the Hundred Years' War, young Amicia de Rune lives a sheltered life on her family's estate, largely separated from her younger brother Hugo, who suffers from a mysterious blood disease. When soldiers of the French Inquisition attack their home and kill their parents, Amicia must flee with Hugo into a countryside devastated by war and overrun by supernatural swarms of plague rats. Alone and hunted, the siblings must rely on each other and a handful of orphaned allies to survive, while Amicia begins to realize that the Inquisition's interest in Hugo may be connected to something far darker than the plague itself.