Historian and broadcaster Tessa Dunlop tells the story of the women of Bletchley Park through exclusive and unprecedented access to the women themselves. The Bletchley Girls weaves together the lives of 15 women who were all selected to work in Britain's most secret organisation - Bletchley Park.

It is their story, told in their voices; Tessa met and talked to 15 veterans, often visiting them several times. Firm friendships were made as their epic journey unfolded on paper. The scale of female involvement in Britain during the Second World War wasn't matched in any other country.

From eight million working women, just over 7,000 were handpicked to work at Bletchley Park and its outstations. There had always been girls at the park, but soon they outnumbered the men three to one. A refugee from Belgium, a Scottish debutante, a Jewish 14-year-old and a factory worker from Northamptonshire - the Bletchley Girls confound stereotypes. But they all have one common bond: the war and their highly confidential part in it.

In the middle of the night, hunched over meaningless pieces of paper, tending mind-blowing machines, sitting listening for hours on end, theirs was invariably confusing, monotonous and meticulous work, about which they could not breathe a word.

By meeting and talking to these fascinating female secret keepers who are still alive today, Tessa Dunlop captures their extraordinary journeys into an adult world of war, secrecy, love and loss. Through the voices of the women themselves, this is a portrait of life at Bletchley Park beyond the celebrated code breakers; it's the story of the girls behind Britain's ability to consistently outsmart the enemy and an insight into the women they have become.