In 1918, when Germany lost the war, it blamed its defeat on the efficiency of Room 40, in the Old Building of the British Admiralty---the room in which German wireless message were intercepted and decoded. German then decided to train a clever spy who would be solidly entrenched in Room 40, whenever the next war came. In 1939 John Usher (Edgar Barrier)is acknowledged the genius of Room 40. His fiancée, Sudan Ainger (Stephanie Bachelor)is also working in Room 40. John's ten-year-old son, David (Bobby Cooper),at school in Switzerland is called home. The night before DAvid arrives home, John is murdered, but his twin brother Robert (Edgar Barrier),impersonates John and continues his work. Only Sir Christopher (C. Aubrey Smith)and David know of the plan. The safety of six high-ranking British officers who are taking a plane to Warsaw, depends on decoding Nazi messages.But the Nazis switch codes and solution seems impossible.