For months, London has been subjected to a German radio broadcast, "The Voice of Terror", announcing acts of sabotage. Sir Evan Barham, of the British Intelligence Inner Council, calls in the famous detective Sherlock Holmes and his partner, Dr. Watson. That same evening, Gavin, one of Holmes' agents, arrives at the detective's home, mortally wounded by a German dagger in the back. Before dying, however, he utters the word "Christopher". Holmes and Watson meet Gavin's wife, Kitty. Learning that her husband has been killed by Nazi spies, she convinces her friends to search the whole of London for the meaning of "Christopher". Holmes and Watson go to Christopher's old docks, but are captured by a group of Nazi spies led by a man named Meade. They are rescued by Kitty's friends, but Meade manages to escape. Holmes tells Watson that he believes there is a spy in the Council. Kitty, who has managed to integrate Meade's gang, tells them that Meade intends to go to Sir Evan's estate that night. As Meade hides in the darkness, Holmes and Sir Evan see a German plane attempting to land, but shots fired by Sir Evan disrupt the rendezvous. Later, "The Voice of Terror" announces a German air attack on the northeast coast of England. Sir Evan insists that British defenses be moved to this area, but Holmes warns the Council that this is a bluff, as the "Voice" always broadcasts at the same time as a German attack, not before. As a Holmes sidekick follows Meade and Kitty's trail to the south coast of England, Holmes forces the Council to go with him. With the support of British soldiers, Holmes captures Meade and a group of German soldiers stationed in an abandoned church. Holmes then announces that Sir Evan is "The Voice of Terror", and that he is in fact a German spy named Heinrich von Bork who has been posing as Sir Evan for twenty years, the real Sir Evan having been executed while a prisoner during the First World War. Holmes then informs the spies that the German invasion force has been destroyed. Meade fires a shot and mortally wounds Kitty, but is killed trying to escape. The British, standing around the murdered Kitty, vow that her death will not be in vain.