Cal Donovan, a theology professor at Harvard, receives an urgent message from a former graduate student, Samia Tedros. Now a museum conservator in Cairo, Samia has discovered a miraculous fragment of papyrus with three unknown lines from the Gospel of Mary Magdalene so explosive that a wealthy man is willing to kill to ensure no one ever sees it.

At the Vatican, another drama is developing. The new Pope has defied centuries of tradition and appointed a nun, Sister Elisabetta Celestino, as his secretary of state. Powerful insiders are outraged that a woman now sits as the second most influential person at the Vatican, and they plot to destroy her.

When Samia shows up at Cal's doorstep in the dead of night, he is drawn into a deadly competition to possess a papyrus with the power to rescue Sister Elisabetta and change the course of Christianity.