In the fourth season of "Das Boot", the brutal submarine war in the Mediterranean Sea comes to a head, while intrigues and secrets spread through Berlin. Resistance to the Nazis grows within the Kriegsmarine's own ranks. After a shared tragedy, the siblings Klaus (Rick Okon) and Hannie Hoffmann (Rosalie Thomass) find their way back to each other. Both fight for their cause. Klaus has returned to the German Reich from Portugal. As a submarine commander he travels to Naples on a secret mission to change the course of the war. Hannie wants to help the many orphans who have become victims of the war and uncovers infamous machinations of the Nazi Reich.