This inspires her to return to her codebreaking past, and she reaches out first to Millie, and then to Lucy and Jean, after unsuccessfully trying to convince the police to follow up her theory about the crimes.

As they all signed orders of secrecy about their work during the war, the two married women (Susan and Lucy), disguise their activities from their husbands as a book club.

Failing to secure police involvement, they move from codebreaking and investigation into the realm of field work, with dangerous consequences on several occasions.

While initially skeptical about becoming involved, Millie, Jean, and Lucy are convinced to help Susan once they realise the lives of many women are on the line.

[9][10][11] In the second series, one character refers to sarin gas as having been developed by the Germans during World War II, along with other such chemical weapons, then taken up by the former Allies.