December 1931.The year is about to turn and one man is holding London to ransom.On the way to see a client on Christmas Eve, 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 an 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 in over, Maisie must negotiate her way through a darkness not encountered since she was a nurse in wards filled with shell-shocked men.