The Roman Catholic Church combats a global rise in demonic possessions by reopening schools to train priests to perform exorcisms. Although nuns are forbidden to perform this ritual, a professor recognizes Sister Ann's gifts and agrees to train her. Thrust onto the spiritual frontline, she soon finds herself in a battle for the soul of a young girl who's possessed by the same demon that tormented her own mother years earlier.

Warsaw, December 1945: the Second World War is finally over and Mathilde is treating the last of the French survivors of the German camps. When a panicked Benedictine nun appears at the clinic one night begging Mathilde to follow her back to the convent, what she finds there is shocking: a holy sister about to give birth and several more in advanced stages of pregnancy.

After the suspicious death of her brother, a priest, Grace goes to the Mount Saviour Convent in Scotland to find out what happened. Once there, she uncovers murder, sacrilege, and a disturbing truth about her past.