Let's start with the Season 3 cliffhanger.
That season ended with the global financial collapse of 1929. It hit Gereon hard and he relapsed in his sobriety, taking a massive dose of morphine. The next season will jump ahead to 1931, but it's likely Gereon will still be struggling with opioid addiction. Season 4 will also continue exploring Gereon and Charlotte's personal relationship — after all, they finally kissed during season 3. Co-creator Henk Handloegten told "Express" that the fourth season will definitely continue that storyline.
As for the crime side of the equation, Handloegten also told Express that Season 4 would closely follow the plot of Volker Kutscher's third "Babylon Berlin" novel, "Goldstein." In the novel, an American gangster named Abraham Goldstein arrives in Berlin and makes a play for control of its criminal underworld, and Gereon investigates him as a favor for the FBI.
However, other sources, such as the German magazine Qiez, have indicated that Season 4 will also include elements from Kutscher's fourth "Babylon Berlin" novel, "The Fatherland Files." That novel finds Gereon investigating a mysterious murder with ties to the rising Nazi party. But whether Season 4 adapts "The Fatherland Files" or not, the Nazi party will definitely be more and more of a problem for Gereon and Charlotte going forward.
