George Downing was one of the English Civil War's most cunning spies. As Oliver Cromwell's head of military intelligence, he forged a career as a politician and diplomat, bribing, blackmailing and browbeating his way to success, before spectacularly betraying his friends by defecting to the Royalist camp. Always close to the action, Downing walked with the most illustrious men of his times: Oliver Cromwell and Charles II were his patrons; Samuel Pepys was his clerk; William of Orange godfather to his son. And when he came to build his famous street, Downing Street, which became the home of Britain's prime ministers, his surveyor was Sir Christopher Wren. Downing helped start two major wars and had a hand in founding New York. Yet he has remained an elusive figure in history, as if hiding himself in footnotes. Turncoat tells the full story of Downing's espionage and double-dealing for the first time, following this mystery man from the asceticism of Puritan New England, across battlefields and through courts, chancelleries and parliaments, to the heart of wealth and power in Restoration London.