This riveting tale begins in 1209 as zealous Catholic knights prepare to storm the walls of the heretic Cathar city of Beziers. "Kill them all," they are told, "God will know his own." The Crusaders took the advice to heart and slaughtered 10,000 people. Two childhood friends find themselves trapped in the horror. One, Peter, is an assistant to the mysterious priest who leads the Crusade; the other, John, with the help of a heretic woman, barely escapes the doomed city with his life. As the brutal war against the Cathars expands, Peter and John are caught up on opposite sides of a search for a secret that might change the world.