The year is 1939. Germany, led by Adolf Hitler, has invaded Poland. The world is about to enter into one of the biggest and most terrible wars in history. But against the might of the Nazi forces rise up courageous spies and codebreakers such as "Garbo" (Juan Pujol), "Agent Zigzag" (Eddie Chapman), Alan Turing and many more. These heroes risked their lives for their countries, often with little or no recognition. Here are some of their incredible stories...