A mysterious code is received at Bletchley and Alan Turing, the chief code-breaker, is unable to break it. He meets the Doctor in a club and when Turing tells him about the code, the Doctor reacts by running away, terrified. Turing confesses his indiscretion to the military and the Doctor is arrested. He eventually succeeds in breaking the code from his prison cell -- the message is a desperate cry for help from mysterious refugees in Vienna.