In this special History Hit film to mark the 80th anniversary of D-Day, Dan Snow meets celebrated military historian Saul David at the National Archives to dig into the top secret documents that tell the story of the incredible planning that went into D-Day.

Hidden in the archives for decades, these papers and photographs reveal meticulous preparations coupled with extraordinary acts of heroism by men and women, researching the Normandy beaches and creating a web of deception that kept German forces guessing where the landings were coming. Using everything from midget submarines to double cross codes, they gave the men assaulting the beaches a fighting chance of success.