When taunting saboteurs warn of a Nazi invasion of the British Isles through their horrific radio menace the Voice of Terror, the British Intelligence's Inner Council calls in Sherlock Holmes to help in the crisis. On the first night of their inquiry, Holmes and Watson find a dying man on their doorstep. His last world sends Holmes to London's seedy Limehouse district, where he enlists the aid of Kitty, the sweetheart of the slain man, to help find the saboteurs.