When Inspector Ben Ross of Scotland Yard one Saturday night at the end of October 1867, London loses an impenetrable yellowish fog. Before she rises in the morning, she lies in the Green Park murdered woman. Allegra Benedict was a beautiful Italian wife of art merchant. But what did she do in London in the afternoon and why did the brooch sold Burlington Arcade just a few hours before her death? How much does the frightened Miss Marchwood know? Is there any connection between the murdered and charismatic priest Joshua Fawcett? And who - or what - is a river ghost that hunts prostitutes working on the banks of the river on foggy nights?