The film describes the sequence of events leading up to the invasion of Norway, especially the Anschluss of Austria, the division of Czechoslovakia after the Munich agreement and finally the invasion of Poland in September, 1939. As a prelude, it justifies the invasion of Norway by outlining the alleged plans of Britain to invade the country, and attempts by the British to mine the leads along the Atlantic coast. When the Royal Navy invaded Norwegian waters to attack the German tanker Altmark and release prisoners held there by the Germans, it signalled an escalation of the growing crisis. The British prisoners had been captured by the German cruiser Admiral Graf Spee during raids on merchant shipping in the Atlantic ocean and Indian ocean in the previous year. The campaign itself opens with the attempt by the German Navy to force entry up the Oslo Fjord, and initially failed owing to Norwegian heavy guns either side of the fjord where it narrowed in the approach to Oslo itself.