Holmes is summoned to the Inner Council of British Intelligence by Sir Evan Barham to help foil a dangerous Nazi spy operation threatening Britain. The spy network is led by an elusive figure known as The Voice of Terror, who broadcasts secret messages via radio. These broadcasts, surprisingly, are recorded in the UK on phonograph records but are transmitted from Germany, making the case complex and deeply rooted in deception.
During an investigation, Holmes's operative Gavin is murdered with a German dagger, whispering the name "Christopher" before he dies. This cryptic clue leads Holmes and Dr. Watson to the Limehouse district, where they meet with Gavin's wife, Kitty. Holmes deduces that "The Voice of Terror" is actually a recording system, using an oscilloscope to analyze sound wave patterns from radio broadcasts. His analysis reveals that the voice being broadcast is a recording, manipulated to create a false persona, and the source is in Germany.
Following a lead from Kitty, Holmes and Watson head to the old Christopher Docks, where they are followed by Sir Anthony Lloyd, Henry-Daniell. The trio is captured by Nazi spies under the command of a man named Meade. Fortunately, some local men intervene, rescuing Holmes, Watson, and Lloyd during a chaotic attack on the spies. Meade, however, escapes through a trap door and boards a waiting speedboat, leaving Holmes and his team determined to track him down.
Kitty goes undercover, pretending to be a thief fleeing from the authorities to gather more intel. She learns that Meade plans to meet with Sir Evan at his estate in Kent that very night. Holmes and Sir Evan, played by Reginald Denny, witness a German plane land at the estate. Sir Evan fires at the aircraft, but Meade manages to pass a valuable package to the pilot, who quickly takes off. Meade then hides, planning his next move.
Holmes's investigation leads him to the South Coast of England, where he, Holmes, and Sir Evan, supported by British troops, finally apprehend Meade and a group of German soldiers stationed in an abandoned church. During the confrontation, Holmes reveals the shocking truth: The Voice of Terror is actually Sir Evan Barham himself, who is an impostor.
Holmes uncovers that during World War I, the real Sir Evan Barham was captured and executed by the Germans. The imposter, Heinrich Von Bock, a German Secret Service agent, had an extraordinary resemblance to Barham. To assume his identity, he was subjected to plastic surgery and closely studied Barham's personal life and mannerisms — even his childhood scar provided a crucial clue. Von Bock had been posing as Barham for 24 years, orchestrating the radio broadcasts to sow chaos and assist German plans for invasion.
The climax reveals that the invasion force has been neutralized, but in a tragic turn, Meade kills Kitty, gravely wounding her in a final act of desperation. Meade himself is killed during his escape attempt. As Holmes, Watson, and Sir Evan stand over Kitty’s body, her heroism is acknowledged, and her sacrifice becomes a poignant reminder of the ongoing struggle against espionage and treachery.
Throughout the operation, Holmes's exceptional deductive reasoning unveils layers of deception and reveals a plot that had lasted for decades. The story underscores themes of loyalty, disguise, and the bitter cost of covert warfare, capturing the suspense and complexity of Holmes’s detective work in a time of war.
