The series starts in the fall of 1942 in occupied France. Nazi submarine U-612 is ready for its maiden voyage, preparing to head into the increasingly brutal warfare with its young crewmen, including the new commander, Klaus Hoffmann. As the 40 young men take on their first mission, they struggle with the cramped and claustrophobic conditions of life underwater. Meanwhile, at the port of La Rochelle, the world of Simone Strasser spirals out of control as she is engulfed in a dangerous liaison and forbidden love, torn between her loyalty for Germany and the French Resistance, and causing her to question everything.
