Set during July 1940, the story opens with 12-year-old evacuee Lydia walking through a village in rural England on a baking hot day. She is wearing a gas mask. The shops and houses are empty, windows boarded up and the village seemingly deserted. Leaving it behind, she arrives at a large house by the marshes -- the house she grew up in. Lydia finds it empty too and her family gone.

Late that night comes Heiden (Jamie Bell), a soldier, gun in hand and heralding a full-blown German invasion. There are, he explains, certain rules she must now follow. He won't kill Lydia, but she cannot leave the house. Is he telling the truth? What is he looking for? Why is he so familiar? And how does he already know Lydia's name?