It's 1649. Three years have passed since midwife Bridget Hodgson and her deputy Martha Hawkins fled York for the safety of the English countryside. But when a mysterious letter from London brings them to the capital, they are forced into service under Jonathan Marlowe, Oliver Cromwell's chief spymaster. Reunited with Bridget's nephew Will, the three seek out Parliament's enemies even as the nation awaits the trial of the King on charges of treason.
Marlowe's first mission for Bridget is to spy upon Katherine Chidley, a notorious political radical and - Marlowe fears - a potential rebel against Parliament. The more time Bridget spends with Katherine, however, the closer the two become, and when Katherine's husband Daniel is murdered, Bridget and Martha join in the search for the killer. As the two uncover Daniel's secrets, the list of suspects grows ever longer, until it includes royalists, radicals, and a figure from Bridget's past, who is equal parts mysterious and deadly. When the killer strikes again, Bridget and Martha realize that he is bent on far more than mere murder, but intends to revive the civil wars or destroy England entirely.