In a remote and crumbling New England mansion, 12-year-old orphan Florence is neglected by her guardian uncle and banned from reading. Left to her own devices she devours books in secret and talks to herself in a unique language of her own invention. By night, she sleepwalks the corridors and is troubled by a recurrent dream in which a mysterious woman appears to threaten her younger brother Giles. Florence becomes convinced that her new governess is a vengeful and malevolent spirit who means to do Giles harm. Against this powerful supernatural enemy Florence must use all her intelligence and ingenuity to both protect her little brother and preserve her private world.