It's a story that will forever change the way you think of the phrase, "Get Out of Jail Free."
During World War II, as the number of British airmen held hostage behind enemy lines escalated, the country's secret service enlisted an unlikely partner in the ongoing war effort: The board game Monopoly.
It was the perfect accomplice.
Included in the items the German army allowed humanitarian groups to distribute in care packages to imprisoned soldiers, the game was too innocent to raise suspicion. But it was the ideal size for a top-secret escape kit that could help spring British POWs from German war camps.
The British secret service conspired with the U.K. manufacturer to stuff a compass, small metal tools, such as files, and, most importantly, a map, into cut-out compartments in the Monopoly board itself.
"It was ingenious," said Philip Orbanes, author of several books on Monopoly, including "The World's Most Famous Game and How it Got That Way." "The Monopoly box was big enough to not only hold the game but hide everything else they needed to get to POWs."
British historians say it could have helped thousands of captured soldiers escape.
So how did a simple board game end up in a position to help out one of the most powerful military forces on the planet? Silk and serendipity.
Of all the tools in a military-grade escape kit, the most critical item was the map. But paper maps proved too fragile and cumbersome, said Debbie Hall, a cataloguer in the map room at the Bodleian Library at the University of Oxford in Oxford, England.
For hundreds of years, even before World War II, silk was the material of choice for military maps, Hall said, because it wouldn't tear or dissolve in water as easily as paper and was light enough to stuff into a boot or cigarette packet. Unlike maps printed on paper, silk maps also wouldn't rustle and attract the attention of enemy guards, she said.
"Initially, they had some problems printing on silk," Hall said. "It's quite technically challenging."
But then MI9, the British secret service unit responsible for escape and evasion, found the one British company that had mastered printing on silk: John Waddington Ltd., a printer and board game manufacturer that also happened to be the U.K. licensee for the Parker Bros. game Monopoly.
"Waddingtons in the pre-war era was printing on silk for theater programs. For celebration events for royalty and that kind of thing," said Victor Watson, 80, who retired as chairman of the company in 1993. "It made a name for itself for being able to print on silk."
He was just a child during the war but said his father Norman Watson, president of the company at the time, worked with British secret service to embed the maps in Monopoly games.
He said a secret service officer named E.D. Alston (known around Waddington as "Mr. A.") used to come by to place the orders in person.
"Because he was in the secret service, I never knew who he was," Watson said.