Christmas Eve, 1931. On the way to see a client, Maisie Dobbs witnesses a man commit suicide on a busy London street. The following day, the Home Secretary receives a letter threatening a massive loss of life if certain demands are not met.

Maisie is invited into Scotland Yard's elite Special Branch as a special adviser on the case - and becomes involved in a race against time to find a man who soon proves he has the knowledge, and will, to murder thousands of innocent people. Before this harrowing case is over, Maisie must negotiate her way through a darkness not encountered since she was a nurse in wards filled with shell-shocked men.