London, 1916. Among a new breed of fiercely independent women are the 'canaries', a raucous group of munitions workers, so-called after their chemically-stained yellow faces. However, a birthday celebration between them is interrupted by a brutal explosion. Unconvinced by the suggestion that this is the work of a German spy, Detective Inspector Marmion and Sergeant Keedy begin their investigations and attempt to untangle the many threads of a surprising case.