In wartime England, cryptic radio broadcasts from Nazi Germany are spreading panic throughout the region.
Sherlock Holmes (Basil Rathbone) and his devoted assistant, Dr. Watson (Nigel Bruce), are summoned by British intelligence to find the source of the mysterious "Voice of Terror." A slain informant's tip sends Holmes to London's grimy Limehouse district.
There the stalwart sleuth gains the trust of the informant's wife, Kitty (Evelyn Ankers), and evades capture by a Nazi spy.