In 1453, seventeen-year-old Luca Vero, accused of heresy and expelled from his monastery, is recruited to help investigate evil across Europe but frees his first subject, Isolde, from captivity in a nunnery, and together they seek the one who defends the boundaries of Christendom and holds the secrets of the Order of Darkness.

Tasked to expose a coin counterfeiting scheme, Luca and Isolde, whose romantic attraction continues to grow, travel to Venice in 1454, where they meet an alchemist who plans to create the Philosopher's Stone, a mystical substance said to be capable of turning base metals into gold and producing the elixir of life.