Mention female spies, and most people think of Mata Hari. But during the Roaring Twenties, Marguerite Harrison and Stan Harding were the cause celebre: two beautiful, accomplished women whose names were splashed across newspapers around the world. Almost a century later, it is easy to understand the fascination with these two remarkable women. Marguerite was a highly respectable and recently widowed American journalist and socialite from Baltimore; Stan was a runaway, a bohemian artist and dancer of British heritage who left her wealthy, religious family to make a life for herself in the expatriate community in Florence. The two women were very different, yet both were strong-willed, independent and highly ambitious women unafraid of taking risks. And both, as the Great War ended and Central Europe dissolved into violent chaos, were looking for adventure.

Their paths first crossed in war-ravaged Berlin during the Armistice and the the Spartacist Uprising in 1919. Fellow travellers, they became friends and, the evidence suggests, lovers. Dodging bullets and interviewing colourful characters in war-torn Europe led these intrepid women, separately, to Bolshevik Russia, a country closed to outsiders since the October Revolution of 1917. Their fateful meeting had repercussions that spanned three decades, involving heads of state and politicians in Britain, the United States and Soviet Russia.

The Lady is a Spy tells their forgotten story: that of two women who, far in advance of their time, worked as foreign correspondents, who operated as spies in dangerous shadowlands of international politics, and who were both imprisoned in Lubyanka, one of the most desperate places on earth. Their lives are reconstructed through numerous primary sources, not only the poems, diaries and letters of their friends and lovers, but also government documents (including newly declassified US State Department papers) that reveal the truth about their espionage careers and - in one case - evidence of a shocking betrayal.