France, 1942: In the submarine port of La Rochelle, the 40-man crew of the U-612 prepares for its maiden voyage. The young men face an unknown mission in the increasingly merciless Battle of the Atlantic during the Second World War. As the new Kaleun, the less experienced Klaus Hoffmann (Rick Okon) is in command - to the annoyance of the older first watch officer Karl Tennstedt (August Wittgenstein). Over the course of the mission, the skills, loyalty and camaraderie of the entire team are put to the test.

At the same time, Simone Strasser (Vicky Krieps) arrives at the base to work as a translator for Gestapo boss Hagen Forster (Tom Wlaschiha). Simone's brother Frank (Leonard Scheicher) is also stationed in La Rochelle. Shortly after their first reunion, Frank asks his sister for a favor that will change her life forever. Soon Simone is forced to question everything she has previously believed.