American teenager Daisy (Saoirse Ronan) is sent by her estranged father away from New York City to the countryside of England to stay with her Aunt Penn (Anna Chancellor). Her distant cousin Isaac (Tom Holland) welcomes her at the airport and drives her home. She is introduced to her cousins, seventeen-year-old Eddie (George MacKay) and young Piper (Harley Bird) and to their friend Joe (Danny McEvoy). However, Daisy is a resentful, needy of love, and aloof girl who believes that she is cursed and that bad things happen wherever she goes since her mother died giving birth to her. Aunt Penn is a busy woman who is studying the war scenario in England, which is on alert due to an imminent terrorist attack, and needs to fly to Geneva. However, the next morning, a nuclear bomb explodes in London and the authorities of the United Kingdom declare a state of siege. Meanwhile, Daisy and Eddie fall in love with each other, but they are separated by the military, which sends girls to one camp and men to another. Daisy and Eddie promise to meet each other again. In a country at war, Daisy and Piper flee their lodging and cross England under martial law, trying to find Eddie and Isaac.