At the beginning of the book German warplanes are flying over cities in Denmark telling them that they have to either surrender and let German forces move though the country or Germany would attack them. The Danish government gave in to German demands to keep their people safe
Many Danish people didn't want anything to do with their German oppressors. There was one boy named Knud Pedersen that founded a sabotage group, that would destroy the Germans things, called the RAF, named after the British Royal Air Force. This group was located in Odense, Denmark.
The only way majority of the Danish got around was by Bicycle. Since the RAF was a bunch of younger kids they couldn't just leave in the middle of the night to attack German supplies they had to be quick and attack during broad daylight, and make sure that no one saw what they had done.