London is plagued for months by the German radio broadcast "The Voice of Terror," which taunts the people of England with tales of German sabotage. Sir Evan Barham of the British Intelligence Inner Council calls upon famed private detective Sherlock Holmes and his associate, Dr. Watson, to help uncover the sabotage ring. Holmes tells the council that he believes that the German broadcasts are merely a "smoke-screen" for something bigger. That night, Gavin, one of Holmes's operatives, stumbles into the detective's flat, fatally wounded with a German dagger in his back. Before he dies, though, Gavin utters the word "Christopher." Later, Holmes and Watson go to the Limehouse district of London, where they meet with Gavin's wife Kitty. Learning that her husband was killed by Nazi spies, Kitty convinces her friends to search all of London to find the meaning of "Christopher." The next day, Holmes and Watson return to the council's office and learn that Sir Evan was almost killed by an assassin's bullet. Holmes tells the council that he has determined that "The Voice of Terror" is actually recorded on phonograph records in England, but broadcast from Germany. Using a tip from Kitty, Holmes and Watson go to the old Christopher Docks, where they are followed by Sir Anthony Lloyd of the council. There, the three men are captured by a group of Nazi spies led by a man named Meade. They are saved, however, by Kitty's friends, but Meade manages to escape through a trap door to a waiting speedboat. Later, pretending to be a common thief, Kitty manages to ingratiate herself with Meade. Meanwhile, Holmes tells Watson that he believes there is a spy on the council, and Kitty later tells them that Meade plans to go to Sir Evan's country estate that night. While Meade hides in the dark, Holmes and Sir Evan watch a German plane attempt to land, but gunshots fired by Sir Evan disrupt the Nazi rendezvous. Later, "The Voice of Terror" announces that there will be a German aerial attack on the northeast coast of England. Sir Evan insists that the British defenses be moved to that area, but Holmes warns the council that the broadcast could be a bluff, as the "Voice" always broadcasts during an German attack, not before one. After one of his operatives traces Meade and Kitty to the south coast of England, Holmes forces the council to go there with him. With the support of British troopers, Holmes captures Meade and a group of German soldiers stationed in an abandoned church. Holmes then tells the stunned gathering that Sir Evan is "The Voice of Terror," and is actually a German spy named Heinrich von Bork who has been posing as Sir Evan for twenty years, as the real Sir Evan was executed as a prisoner of war during World War I. Holmes then informs the spies that the German invasion force has been destroyed. The angry Meade shoots and fatally wounds Kitty, but is killed himself as he attempts to escape. The British gentlemen then stand around the murdered Kitty and swear that her heroic death will not be in vain.