The detective “Holmes,” portrayed by Basil Rathbone, doesn't have a completely convincing German accent, but he still brings Swiss scientist “Tobel” (William Post Jr.) back to London. Here, it is revealed that Tobel created a bombsight with the potential to significantly impact the war effort for the allies. This scientist seems rather mysterious and refuses any security measures, so it’s not unexpected that he's kidnapped shortly after arriving. He was wise enough to divide his invention into separate parts, which sets off a chase between Holmes, his dependable companion “Watson” (Nigel Bruce), and their most dangerous opponent yet, “Prof. Moriarty” (Lionel Atwill). Driven to locate his adversary, Holmes ventures into the fog searching for the captive, but his enemy has already predicted many of his actions. Fortunately, “Watson” and a surprisingly competent “Lestrade” (Dennis Hoey) are available to assist in stopping this wicked scheme, not only for the country's sake but also for their friend’s well-being. The film carries a more obvious wartime theme, clearly identifying Nazis as the ones who would gain from such an act of betrayal. Throughout the story, there are hidden areas, some physical injuries, and a clever attempt at murder targeting the pair, making it an undemanding hour of mystery to enjoy. I do have one small complaint—they repeatedly call Britain “England.”

Source on TMDB