It is 1936 and Spain is on the brink of civil war. Across Europe, young men are enlisting in the International Brigade to free their Spanish brethren from the grip of fascism, leaving sisters and lovers at home.

But not all women are content to be left behind. In Britain, Charlotte McBain and Libby Forbes, friends from opposite sides of the class divide, are determined to do what they can; in Spain, Rosita García Díaz, fiercely loyal to her family and country, cannot stand by and watch. Three brave women, inspired by patriotism, idealism, love and even revenge, dare to go into battle against tradition and oppression.

Tying them all together is Jo, Libby's granddaughter. Five decades later she travels to Spain hoping to make sense of a troubling letter hidden among her grandmother's possessions. What she learns will change all of their lives forever.

Deceit, heartbreak and a longstanding fear of reprisals must all be overcome if the deeds of the forgotten women are to be properly honoured.