David Olusoga traces the lives of residents of two apartment blocks — Montagu Mansions in Marylebone, London, and 72 Pfalzburger Strasse in Wilmersdorf, Berlin — to tell the story of the world's deadliest conflict, the Second World War. He starts his journey back in the 1920s, examining how the events of the First World War changed the lives of the residents and sowed the seeds of an even greater conflict two decades later. As the Nazi Party emerges, some of the families are placed in terrible danger.