During World War II, on the German-occupied island of Jersey, Grace, a single woman awaiting the return of her husband, lives on her estate with her two children, nine-year-old Nicholas and eleven-year-old Anna. - who suffer from photophobia, a rare disease that prevents them from being in the sun. The house is completely dark and there are strict rules: no door can be opened without first locking all the others. The distant island of Jersey fills Grace with fear. The heroine feels safe only at home. However, this security is only apparent. As time goes on, Grace has more and more questions to which she cannot find answers. One day, Grace finds out that there is someone else in her house besides her and the children.