For almost five millennia, indigo - a blue pigment obtained from the small green leaf of a parasitic shrub - has been at the centre of turbulent human encounters, prized by slave traders, religious figures and the fashion world.Indigo is the story of this precious dye and its ancient heritage: its relationship to slavery as the 'hidden half' of the transatlantic slave trade, its profound influence on fashion, and its spiritual significance, which is little recognised but no less alive today. It is a richly told story, brimming with electrifying tales of those who shaped the course of colonial history and world economy.But this is also the story of a personal quest: Catherine McKinley's ancestors include a clan of Scots who wore indigo tartan, several generations of Jewish 'rag traders' and Massachusetts textile factory owners, and African slaves who were traded along the same Saharan routes as indigo, where a length of blue cotton could purchase human life. Her journey takes her to nine West African countries and is resplendent with powerful lessons of heritage and history.