When the communists took over in Czechoslovakia in 1948, many people decided to leave. At the same time, the Americans were sending armed freedom-fighters into the country. The communist reaction was to seal the borders with the West, creating the Iron Curtain. This, of course, took time and there was a period when it was still possible to walk across the border if you knew how. There were local people who were prepared to help escapees and the most famous of them was the "King of Boehmerwald".
He was armed and an excellent shot, shooting accurately while falling to the ground. A prototype for Rambo, perhaps. They never got him and when the border was finally sealed and it was impossible to get near the border on the Czech side (there was a several miles-wide forbidden strip of land), he simply stayed in the West.
The film tells us otherwise. It pitches Czechoslovak border guards, ordinary drafted soldiers selected for their communist persuasion, in an uneven battle against this man. They do not even know who he is but they prevail in the end. It is an exciting film.