1943, Brisbane: The war continues to devastate and the battle for the Pacific threatens Australian shores. For Ellie O'Sullivan, helping the war effort means utilising her engineering skills for Qantas as they evacuate civilians and deliver supplies to armed forces overseas. Her exceptional logic and integrity attract the attention of Central Bureau - an intelligence organisation working with England's Bletchley Park codebreakers. But joining Central Bureau means signing a lifetime secrecy contract. Breaking it is treason. With her country's freedom at risk, Ellie works with a group of elite women who enter a world of volatile secrets; deciphering enemy communications to change the course of the war. Working under immense pressure, they form a close bond - yet there could be a traitor in their midst. Can the women uncover the culprit before it's too late? As Ellie struggles with the magnitude of the promise she's made to her country, a wedge grows between her and those she holds dear. When the man she loves asks questions she's forbidden to answer, how will she prevent the double life she's leading from unravelling?