When young Will Campbell joins the most powerful organization in Europe, The Order of the Knights Templar, he finds himself drawn into a world of intrigue and danger. He is charged with recovering a heretical book stolen from the order’s vaults—but what Will doesn’t know is that the book, in the form of a Grail Romance, hides the covert plans of a secret group within the Temple known as the Anima Templi: the Soul of the Temple. Whoever controls the book controls the fate of the Templars—and it seems that everyone around Will is ready to kill to possess it.

Brethren also traces the rise of Baybars Bundukdari, an ambitious commander in the Egyptian army, who, after assassinating the sultan, takes control of Egypt and Syria. The two stories come together during Baybars’s campaign for a new Holy War that will cripple an empire and bring the Crusaders to their knees.