In the occupied French port of La Rochelle in the autumn of 1942, U-612 is preparing for its first voyage led by Capt. Klaus Hoffmann, who feels pressure to live up to his father's legacy; Simone Strasser begins work as a translator.

Aboard U-612, the men are adjusting to their new lives as a tense disagreement arises; in La Rochelle, Simone finds herself caught between Forster, mysterious Carla Monroe, and the entanglements her brother left behind.

Following a mission completion, the U-612 crew is reeling from a tragic death, adjusting to new passenger Samuel Greenwood, and witnessing further strain among the leadership; on land, Simone continues to uncover shocking discoveries.

The La Rochelle residents struggle to recover following an explosion, and Simone finds herself torn between alliances to Forster and Carla; on U-612, tensions are at a breaking point following a disastrous confrontation with the enemy.

As the crew celebrates the departure of one passenger, the reappearance of another is startling; on land, Carla tries to convince Simone that there are sacrifices to be made in the battle for power, and she starts to plot her next move.

Trapped below the angry, ominous rumble of a destroyer, the outlook is bleak for the U-612 crew; they must work together to find a way out with the odds stacked against them; in La Rochelle, Simone begins to come to terms with her new double life.