S1, E1: England is alone - a Protestant nation in a largely Catholic Europe. Then, 12 years into Elizabeth's reign, the pope declares her a heretic, which in the hearts of England's Catholics gives them permission to kill her. Queen Elizabeth looks to her spymaster William Cecil to stop the Catholic assassins getting through. Cecil establishes a huge espionage network - England's first secret service. His spies break Catholic conspiracies at home and abroad. Cecil's network is put on high alert by intelligence from a source in Catholic Europe. As a result he catches a courier carrying coded letters that lead Cecil to unravel a plot to assassinate Elizabeth and install her cousin Mary, Queen of Scots on the English throne.