London. A mysterious serial killer brutally murders young blond women by stalking them in the night fog. One foggy, sinister night, a young man who claims his name is Jonathan Drew arrives at the guest house run by the Bunting family and rents a room.
A film projectionist longs to be a detective, and puts his meagre skills to work when he is framed by a rival for stealing his girlfriend's father's pocketwatch.
Carter Holmes, master criminologist, must help the oft-kidnapped Ruth Stanhope to find the 9 daggers that will unlock the secret of the cursed Devil's Trademark!
When a couple of scammers hold young Alice Faulkner against her will to discover the whereabouts of letters whose dissemination could cause a scandal affecting the royal family, Sherlock Holmes decides to take over the case. (Considered lost, a copy was found in 2014, in the vaults of the Cinémathèque Française.)