London, October 1941. Government message runner Freddie Hackett witnesses a murder in a bomb-damaged street and later sees the killer at the office where he delivers a sealed packet. The police dismiss his account, but Maisie believes him and begins an investigation that immediately overlaps with her confidential war work.
Maisie is assessing recruits for clandestine operations in occupied France, and the suspected killer has links to competing British and Free French intelligence interests. Evidence leading back to the First World War tests Freddie’s reliability and Maisie’s loyalties, while a private crime threatens people and operations far beyond the London street where it began.