With the Luftwaffe attacking everywhere, the barrage balloon defense is extended to Scotland in this informational short subject.
If you've looked at movies of London during the Blitz, you'll have seen barrage balloons, looking like puffy torpedoes floating above the houses. The balloons themselves were not intended as part of the defense. Rather it was the steel cables that anchored them in place. They were not intended to protect against bombing. Instead, the theory went, they would stop the fighter planes from making runs against people in the street. To do so, they would have to descend below the balloons, and if they did, they risked running into one of the cables, destroying the plane.
How effective were they? I have no idea. However, the movie is a good handling of the subject, told in that calm, matter of fact manner that so many British propaganda and information films used in this period.