The Rose Code

by Kate Quinn

Until D-Day, the fatal day, when they had splintered apart and become two girls who couldn’t stand the sight of each other, and one who had disappeared into a madhouse.

I knew from the beginning that The Rose Code would be the story not just of codebreakers at Bletchley Park, but of a broken friendship. Writing a dual timeline carries an inherent conundrum: if you are showing the same characters in a later time, it removes some of the mystery from the story because it's obvious that these women survived their war. So I introduced a different kind of mystery in the later timeline, and that question pulls tension throughout the book: what happened to these three friends during the war to make them turn against each other?