England, December 1942.
Only on foot would you notice the entrance, partially concealed by a row of laurels, to a gothic villa hidden deep in the country near Bletchley Park. This is 'Black' HQ, an undercover organisation not officially recognised by the wartime government. Sam Thayer, Black's creator, knows that it's not chasing codes that will change the course of the war, but what is done with the decrypts. Vivienne Thayer, working alongside her husband, is having a good war: there's no need for austerity when you have secret channels throughout Europe and she has an indulgent new lover in her colleague, Frido, a German member of the Berlin resistance. Psychological warfare, sowing doubt, generating fear - this is what the small team at Black work on. But there is doubt and fear within the villa that will threaten everything.
Meanwhile, in London, there are many dangers in the darkness of the blackout, rubble lying around, holes yawning open, figures lurking, and waiting. In the middle of this chaos, three women from very different backgrounds are brought together by their work for the War Artists Advisory Committee. Laura Knight is the most famous female artist of her day but at the age of sixty-six, will she ever find a subject worthy of her talent? Cecily Browne is getting married in ten weeks, but with her fiancé one of a RAF bomber crew, will there even be a wedding? Faith Farr desperately doesn't want to be found - especially by her husband - but why is she so afraid, and what is she hiding?
As the darkest hour of the conflict approaches and the lives of both groups collide, only one thing is certain: no one can be trusted.
