A sex killer haunts the afternoon fog of 1930s London. He is on the prowl, looking for a schoolgirl. This monster, in turn, is being hunted. Detective-Inspector 'Dick' Turpin understands the psychology of his prey, but is more adept at finding habitual criminals than sex murderers. So it is up to formidable do-gooder Miss Asta Thundersley to poke her nose into the investigation. She is a habitue of the Bar Bacchus and believes one of the regulars is the killer, invites them all to a party and laces each one with alcohol to loosen their tongues.