Set against the backdrop of post-war Germany, Der Illegale follows the story of a young man named Stefan who, after being arrested for deserting the army, finds himself in the country’s illegal labor market. As he attempts to start a new life, he faces not only the challenges of survival but also a world where trust is rare, and every action carries immense risks. Through Stefan's journey, the film explores themes of power, survival, and the lengths one will go to in order to escape the clutches of an unforgiving system. The Illegal is a gritty and realistic look at the underbelly of life in a divided Germany during a time of political tension and social upheaval.

Die Handlung basiert auf wahren Erlebnissen eines Spions, der zu Beginn der 1950er-Jahre vom sowjetischen Geheimdienst in die Bundesrepublik eingeschleust wurde, um dort einen Agentenring aufzubauen.