The year is 1939. Germany, led by Adolf Hitler, has invaded Poland. The world is about to enter into the biggest and most terrible war in history.

But from the depths of despair rose some very courageous, selfless individuals, such as 'Garbo' (Juan Pujol), 'Agent Zigzag' (Eddie Chapman) and Alan Turing, who risked their lives for the sakes of others in need, often for no gain and with no recognition.

Here are some of their incredible stories...