When Harriet Grace discovers her grandfather is going blind and unable to operate the railway signal box, she decides to go against the rules and man the signals herself. This will set off a spiral of events when her deception is discovered and the authorities are informed. Harriet must attend a Board of Trade Inquiry and explain her actions. Forced to leave their small cottage, Harriet and her grandfather move to York to live in the same boarding house as the man she loves, Edward Hainsworth, a locomotive fireman. But her move to York will be the catalyst for many changes in her life including the opportunity to work as a telegrapher and clerk with the York Constabulary. Meeting handsome Detective Constable Mackinnon Taylor brings her mixed emotions. And then on a visit to the signal box, Harriet intercepts a strange coded telegraph message leading to the discovery of a shameful secret, the implications of which will threaten the lives of Ned and herself as they pursue an abduction and possible murder.