"You are to report to Station X at Bletchley Park, Buckinghamshire, in four days time ... That is all you need to know." This was the terse telegram hundreds of young women throughout the British Isles received in the spring of 1941, as World War II raged. As they arrived at Station X, a sprawling mansion in a state of disrepair surrounded by Spartan-looking huts with little chimneys coughing out thick smoke--these young people had no idea what kind...