In her remarkable new book, Alison Weir recounts one of the greatest and most remarkable love stories of medieval England. It is the extraordinary tale of an exceptional woman, Katherine Swynford, who became first the mistress, and later the wife, of John of Gaunt, Duke of Lancaster. But Katherine Swynford was much more than that. She was the mother of the Beauforts, and through them the ancestress of the Yorkist Kings, the Tudors, the Stuarts and every other sovereign since - a prodigious legacy which has shaped the history of Britain.