Four-year-old Annette and Lisette Balfour are sent to live with their aunt's family in Scotland after their mother dies in the Spanish flu epidemic of 1919, and their father goes back to sea. But times are hard for farmers, and when an opportunity arises for Lizzie to move to London with her mother's wealthy sister, the family seize it - even though it means the twins will be separated. As they grow up, Annie and Lizzie live very different lives, one in solitary luxury in London, the other, surrounded by loving cousins, on a struggling Scottish farm. Their experiences drive a wedge between them, but the bond between the twin sisters is a deep one. When the Second World War comes, Lizzie trains as an agent sent to occupied France where she brings deadly danger to the French relatives of her late mother's glamorous sister. Annie joins the Wrens and learns of her father's new life with his Japanese wife and son in Singapore under Japanese occupation. Over the years, their love survives hardship, tragedy and the horrors of war, to emerge stronger than ever.