The fascinating story of an Edwardian husband swap, against a backdrop of suffragettes, seances, politics and the birth of the supposed new Messiah, from award-winning historian, Susan Pedersen.

'BURN THIS!' Why did so many letters between Lady Frances and Lady Betty Balfour begin this way? What did they have to hide?

Alike in so many ways - political, passionate, argumentative and deeply intelligent. It's no surprise that once Frances and Betty became sisters-in-law, they became close friends. There was just one problem. Frances was in love with Betty's husband. Their unconventional solution they found was an 'experiment in living', a menage a quatre. Setting up homes on the same street, they shared everything: money, meals, governesses, and husbands.

When Susan Pedersen discovered their amazing cache of letters, she was spellbound.

'We tend to see elite women of this era romantically - we wonder whom they will marry and track that marriage plot - but we don't always follow them into their marriages, as they struggle to live consequential lives. Marriage isn't the end of women's lives. It can be, as it was for Frances and Betty, the catalyst to political activism.'

Her book follows their extraordinary friendship as both women seized every freedom afforded to them by the changing times, leaving their drawing rooms for the streets and the soapbox, joining the fight for the women's vote.

This is the untold story of two women - radicals, rivals, sisters - who carved a path together through an elite male world, sharing their ideals, their heartbreaks, and most of all, their secrets.