Frinton-on-Sea, 1940. Impatient 19-year-old Cathy Raine (Harriet Walter) is expected to see out the war on the production line at the local munitions factory. Against the wishes of her strict father and dull fiancé, in a bid for freedom and self-fulfilment, she signs up for the ATS. In her soldier's training she befriends working-class Mary (Brenda Blethyn), and finds work at a listening station transcribing coded German transmissions. Moving on to the code-breaking centre at Bletchley Park, Cathy meets Turing-like mathematics don John Turner (Nicholas Le Provost) and is attracted by his endless curiosity. Circling ever closer to core operations, Cathy is kept in the dark by the secretive male hierarchy until she stumbles upon a secret signals intelligence file that may jeopardise her safety.