Summer 1937. What could be more fitting in the cool afternoon of an English country lane than a group of cycling tourists steadily pedalling their way from one historic site to another, stopping to camp overnight in fields along the way.

The only problem was, that summer, some of those groups of teenage boys were Hitler Youth.

In an era without satellite photography, when detailed ordnance survey maps could be hard to come by and when tension in Europe was rising, MI5 were worried that this innocent cyclo-tourism was a cover for spying.

MI5 had been told that Hitler Youth groups visiting abroad were asked to complete a detailed questionnaire, including questions on terrain, population, and political views of the population.

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