With London's streets deserted, a young flower girl is murdered. But why was she out so late on the foggiest night of the year? Inspector Witherspoon of Scotland Yard has nowhere to even begin looking for a suspect. However, Mrs. Jeffries, his housekeeper, is even more accomplished at detection than she is at household management. A policeman's widow herself, her quick wits allow her to nudge the inspector in the right direction to solve the mystery.