Over a decade before Margaret Thatcher swept to power, another woman was running Britain from 10 Downing Street: Marcia Williams was the first ever female political adviser to a Prime Minister and was said to have a powerful grip on her boss. Historians have described the relationship between Marcia and Labour Prime Minister Harold Wilson as one of the most famous but mysterious partnerships in modern political history. A brilliant tactician, Marcia masterminded Wilson's multiple election victories. Indeed, he said that but for her ingenuity, he would never have become Prime Minister. But misogyny, jealousy, a shocking private life and accusations of money-grubbing and bribery all contributed to her reputation as a public nuisance. Marcia's young and ambitious male colleagues said she humiliated and screamed at Wilson, single-handedly ruining his chances of being remembered as a 'great' Prime Minister. There is no doubt Marcia was outspoken, forthright and, by contemporary standards, deeply unconventional. But her critics failed to understand her unbreakable partnership with Wilson - they were politically wedded to each other and equal contributors to his success. In this passionate and fascinating biography, Linda McDougall seeks to rescue Marcia from previously dismissive verdicts, suggesting a more nuanced context in which to assess her actions and reactions and restoring this trailblazing pioneer to her rightful place in British political history.