In 1918, at the end of World War I, German High Command officials blame much of the German defeat on the work of the British Admiralty's decoding unit, known as "Room 40," which intercepted and decoded German wireless communications. Vowing to avenge the defeat in the next war, the High Command places a German spy at the British decoding office in London. Twenty-one years pass, and Germany, again at war, is aided by the services of their secret spy. At the offices of the Room 40 operation, cryptographer John Usher learns that his son David, a young boy studying at a boarding school in Germany, is being forced by the war to return to England. Before leaving Germany, David bids farewell to his German friend, Carl Eberling, and promises to write his friend in code. John arranges for his fiancée, Sudan Ainger, to begin work for the decoding department, and while she is being introduced to her new colleagues, Col. Hedley, Roylott Bevan, Mr. Mason, Mortimer Cope and Mr. Waterlow, John is given orders from his supervisor, Sir Christopher Pelt, to decipher the new German code. Late one night, after successfully deciphering the German code, John is murdered. His body is found the next day by Pelt, who keeps John's death a secret and immediately telephones John's twin brother Robert. Assuming that John was murdered by one of his colleagues, Pelt asks Robert to impersonate John in the hope that the murderer will reveal himself when shocked by the sight of the man he thought he had killed. Pelt's plan proves ineffective, however, when all of John's colleagues, including Sudan, show an ordinary reaction to Robert's presence. Determined to find his brother's killer, Robert continues his impersonation, which necessitates bandaging his left hand to hide a dissimilarity between him and his dead twin, and launches an investigation into each one of John's colleagues. When David arrives in England, Robert tells him that his father has died, then enlists the boy's help in keeping his identity a secret. Over the course of his investigation, Robert makes an unsuccessful attempt to link one of John's colleages to the notorious German master spy known as Josef. More complications arise when Pelt is murdered and Robert is again unable to find a suspect. A short time later, after Waterlow is appointed to replace Pelt, the British decoding unit is assigned to decipher a new German code. Robert and the other members of Room 40 are baffled by the new code until Robert learns that David and his young friend in Germany have been using the same type of code to write to each other. Realizing that Carl must have found the code among the papers belonging to his father, a German cryptographer, Robert searches for the key to break the code. His search takes him to the London restaurant operated by Josef, who discovers Robert's mission and holds him at gunpoint. Robert manages to escape, however, and returns to Room 40. Soon after Robert discovers that the Germans are planning to blow up an airplane carrying British officers to Warsaw, Waterlow points his gun at him and reveals himself to be the German spy and the man who murdered John and Pelt. Robert, who has a gun hidden in his bandaged hand, shoots and kills Waterlow, and then calls the British authorities in time to prevent the explosion of the airplane carrying the high-ranking officers.