When Inspector Ben Ross of Scotland Yard walks home one Saturday night in late October 1867, London is covered in an impenetrable yellowish fog. Before he gets up in the morning, a murdered woman lies in Green Park. Allegra Benedict was the beautiful Italian wife of an art dealer. But what was she doing that afternoon in London, and why was she selling a brooch in the Burlington Arcade just hours before her death? How much does the frightened Miss Marchwood know? Is there any connection between the murdered and the charismatic priest Joshua Fawcett? And who - or what - is the River Ghost, hunting prostitutes working on the banks of the river on foggy nights?