London prostitutes are mortally frightened: in the thick fog that envelops the city, they are attacked by a maniac. In a bloody shroud, he hunts for unfortunate girls near the Thames, so he is nicknamed the River Spirit. After one of these foggy evenings, a woman's body is discovered in Green Park, but she turns out to be the missing wife of a wealthy antiques dealer. Suspicion falls on several people, including the River Spirit. Inspector Ross and his unusually smart wife Lizzie begin an investigation...