One night during a heavy rainstorm, a stranded stranger knocks on the door of Angela's country cottage, claiming his car has broken down. Angela lets him in to shelter but soon a police officer arrives, asking questions, amid conducting a manhunt for an escaped criminal. Angela puts two and two together, but is there more to it than meets the eye?

In 1940s, the nuns of a remote Scottish island convent ready for an impending storm, but when a baby is left on their doorstep, Sister Agnes becomes convinced he is the Devil and wants to kill the child. After Agnes is locked away a grim series of events ensue as the storm rages, and as her Sisters begin to question faith and reality, Sister Eleanor must try to repent and survive the dark events at St Augustine’s.