London is shrouded in a pea-souper fog. Wandering in that fog, for reasons not fully established, was the beautiful Allegra Benedict, wife of an art dealer. When the fog clears, her murdered body is discovered in Green Park. How much does frightened little Miss Marchwood know? Is there any connection with charismatic preacher, Joshua Fawcett? Who - or what - is the River Wraith who preys on the prostitutes working the riverside on foggy nights? Lizzie Martin and Ben Ross are now married and have set up home near to Waterloo Station. Ben, officially, and Lizzie, unofficially, must investigate.