As I grew to know Sally better, and as she grew up, I realised that she'd naturally become interested in social questions of every kind. She is a modern woman. And one of the biggest issues of that time - the book is set in 1881 - was the great increase in Jewish migration, following the persecutions in Russia. So that forms the background: these shiploads of suffering, exploited people drifting westwards in the hope of a better life, and all the criminal gangs making money out of swindling them, and the politicians denouncing them - why, it could be taking place today. But Sally herself, and a mystery, is at the heart of the story.