What becomes of your life when everything you thought was solid and certain about it turns out to be a fantastically complicated lie? This is the quandary faced by Ruth Gilmartin (Michelle Dockery) when, one day in 1976, her mother Sally (Charlotte Rampling) suddenly reveals that she has been living a double life. She is not the respectable Sally Gilmartin, but in fact Eva Delectorskaya, a spy for the British Secret Service who has been on the run for thirty years.