An elderly widow is arrested for treason, and shares the story of her life as the KGB's longest serving agent in the British government. Recruited as an idealistic radical in the 1930s, she spent decades providing the Soviet Union with details of Britain's nuclear weapons - in the hope that neither nation would gain the edge in the arms race. Espionage drama based on Jennie Rooney's novel, starring Judi Dench, Sophie Cookson and Nina Sosanya