A photographer in 1889 London, Miss Sarah Bain runs a private detective agency with her friends, Lord Hugh Staunton and former street urchin Mick O'Reilly. Their sole credential is that they solved the Jack the Ripper case, a secret they can never tell because they did it outside the boundaries of the law. Their new big case arises when a wealthy banker, Sir Gerald Mariner, posts a handsome reward for finding his missing infant. All of London joins in the search. But Sarah has an advantage--a photograph she took during a routine surveillance job, which unexpectedly reveals a clue about the kidnapping.