Germany in the summer of 1961 - the "Iron Curtain" divides the country. Only in Berlin is the border still permeable. West Berlin is the open wound of the GDR. Until August 13, 1961, a summer Sunday that would divide the world into a before and an after. 2011 marks the 50th anniversary of the day on which the division of Berlin cemented the division of Germany and Europe for more than two and a half decades. The docu-drama "Geheimsache Mauer - Die Geschichte einer deutschen Grenze" by Christoph Weinert and Jürgen Ast tells the story of the Berlin Wall and the inner-German border from a new, unusual perspective: from the point of view of those who planned, built and guarded it. The film takes the viewer behind the scenes of the Wall builders: it reveals the "concreted" thinking and calculating calculations of the Wall strategists - and their secret plans to perfect the deadly border further and further.