Harlan and Alfred Braun, who also worked on the screenplay, based the film on the autobiography of Joachim Nettelbeck [de; pl], mayor of Kolberg in Pomerania, and on Paul Heyse's later play adapted from the book.

)[2] The film recounts the defence of the besieged fortress town of Kolberg against French troops between April and July 1807, during the Napoleonic Wars.

In fact, the city's defence, led by then-Lieutenant Colonel August von Gneisenau, held out until the war was ended by the Treaty of Tilsit.

The opening scenes show Prussian Landwehr and volunteers marching in the streets of Breslau through enthusiastic crowds.

Nettelbeck struggles to defend his city against others' cowardice and lethargy, and the old-fashioned ideas of the garrison commander.

Finally, having been threatened with execution by the military, and convinced that Kolberg can be saved only if a great leader can be found, Nettelbeck sends Maria, a farmer's innocent daughter[2] the dangerous journey to Königsberg, whither the Court of Prussia has retreated.

Gneisenau convinces Frederick William to do so; he writes the proclamation An Mein Volk ("To my People") announcing the Wars of Liberation.

[6] Kristina Söderbaum, the director's wife, was cast as Maria, a farmer's daughter and one of the lead roles in the film.

Harlan stated that he was given 4,000 sailors for a scene of a French attack across flooded fields after appealing the Kriegsmarine's rejection of his demand.

[7] The film's extras amounted to perhaps 5,000 soldiers; hundreds of Kolberg people participated for a daily fee of 5 ℛ︁ℳ︁.

The number of extras is commonly exaggerated at 187,000, and claims of entire divisions of troops taking part are completely false.

[5][failed verification] It was also screened in the Reich Chancellery after the broadcast of Hitler's last radio address on 30 January.

[13] The city of Kolberg was declared a Festung ("fortress town") as Soviet forces neared it on 24 February 1945.