London, 1864. Orphan Sarah O'Reilly has disguised herself as a boy so that she can work in the offices of the London Mercury newspaper. Here she meets Lily, a wealthy widow who writes a column for the paper. Lily has been enlisted by Lady Cynthia Herbert to help catalogue her magnificent jewel collection. The gems include a fiery red diamond that seems to exert an unsettling influence over anyone who touches it. Then two gruesome murders take place. A local simpleton, Holy Joe, is blamed for the murders but neither Lily nor Sara is convinced.