It portrays the Battle of Westerplatte between the Polish Army and Nazi German forces at the start of World War II in 1939.

The film focuses on the conflict between Westerplatte commander Henryk Sucharski (Michał Żebrowski) and his deputy Franciszek Dąbrowski (Robert Żołędziewski).

[1] The film has sparked some controversy with allegations saying it was "anti-Polish" and "unpatriotic" in its revisionist attitude to the heroic myth of the Battle of Westerplatte which became a symbol of the against-the-odds defiance to the Nazi invasion, and a group of historians even dismissed it as a false depiction of the battle that is being "offensive to the dignity and honour of Polish soldiers.

"[2][3] According to The Telegraph, "while the film has garnered its detractors, cinema critics have dismissed claims that The Secret of Westerplatte belittles the soldiers who fought and died in the battle.

'"[3] Possibly in an attempt to deflect the negative criticism, which the title word "Tajemnica" (Secret), implying as it does that the film makes revelations and runs against the long-held perception of the battle as an heroic Polish defence, publicity posters omitting the word were produced[4] and at least three other versions of the title have been used.