Formerly called Sherlock Holmes Fights Back this fourth installment in the Basil and Nigel series continues the wartime theme against the Nazis. In a race to solve Doyle's secret code borrowed from The Adventure of the Dancing Men, Holmes is pitted once more against his arch enemy Professor Moriarty who, played by Lionel Atwill, seems to have gained allegiance with the 3rd Reich! Atwill also appears as Dr. Mortimer in The Hound of the Baskervilles (1939).
A newcomer to the series, actor Dennis Hoey, makes his first appearance as Scotland Yard Inspector, Lestrade. He would continue in this role for a total of six films!

Also new on the set was Director Roy William Neill, who would bring his talent behind the screen in all twelve of the remaining films. Credited as a master of lighting effects, he would encourage better pacing, and atmosphere throughout his direction.