Daisy (Saoirse Ronan), a teenage New Yorker, is sent to England one summer to stay with cousins she has never met before. Immersed in a dreamy pastoral idyll she soon falls madly in love with her cousin Eddie (George Mackay). Their summer is blown apart by the sudden outbreak of a 21st century World War when, along with Eddie’s younger siblings Isaac (Tom Holland) and Piper (Harley Bird), they are forced to fend for themselves. Eventually separated, Daisy is forced on a terrifying journey to be reunited with the boy she loves. Eddie is never far from Daisy’s mind but as her journey to find him gets more desperate, the idea of what she may or may not find begins to consume and haunt her.