An inexperienced U-boat crew are plunged into a perilous secret mission, while new translator Simone starts work at the base.

Unity on U-612 has crumbled and Hoffman and Tennstedt's continued conflict could jeopardise the mission. On land, Simone wrestles with a difficult choice.

August 1943: Franz Buchner is in action with the U-949. An informer on board divides his crew. The Hoffmann family comes together in Berlin to attend the premiere of a film about Klaus.

Robert Ehrenberg supports the resistance within the navy and gives Klaus Hoffmann a tip. Bobby Schulz is on his trail. U-949 comes under fire in the Strait of Gibraltar.

Commander Buchner reaches the coast of Naples. Klaus Hoffmann is also on his way there. His sister Hannie meets twelve-year-old Hertha.

Klaus Hoffmann commands the dilapidated U-330 on a secret dive, but not all men trust him. Hannie wants to take Hertha in. Staff officer Heinrich Gluck has to make a decision.

After a risky maneuver, the U-330 appears next to a US Navy ship. SS man Koch reveals the identity of the spy “Fidelio”. Hannie tracks down the Lebensborn organization.

Schulz kills with his own hand for the second time. Klaus Hoffmann realizes that the Allied invasion has long been underway. A man is waiting in Naples with a murder order.

Tennstedt and Hoffmann continue to butt heads aboard U-612, while rebel Carla Monroe plans a mission in La Rochelle.

A surprise new arrival takes Greenwood's place, causing a divide. Plus, Carla and Simone grow closer and a resistance fighter arrives at the safe house.

Things look bleak for the crew of U-612, as they desperately try to work out their next move. Simone adjusts to her new position between Carla and Forster.

The U-boat crew struggle with recent events and what lies ahead. A revelation in La Rochelle infuriates Forster spelling bad news for Simone.

Hoffmann insists on further training despite the crew's desire to head to battle. In La Rochelle, Simone pieces together the previous night's events.

Forster reveals to Simone that U-612 has gone missing. Meanwhile, the resistance endanger the innocent to further their cause.

Hoffmann struggles to hang on as his chance to escape heads toward US shores. Margot pushes on with her plans.

Tensions build beneath the ocean as Wrangel continues his pursuit of U-822. In New York, Sam takes drastic measures.

Johannes and Wrangel try to bring their crews into line. Despite his growing love for Cassandra, Hoffmann seeks an escape route.

Hoffmann struggles to hang on as his chance to escape heads toward US shores. Margot pushes on with her plans.

Spring 1943: L.I. Ehrenberg prepares submarines for use in Kiel. Minors are recruited as crews. Wilhelm Hoffmann attends a reception at the request of his son-in-law.

Forster is investigating the village man murder case in Lisbon. In Kiel, the new sailors of the U-949 get into battle. Royal Navy captain Swinburne is travelling to Liverpool for his son's funeral service.

The U-949 is in danger during a test drive due to Commander Buchner's orders. Forster comes across a trail in a casino and makes the acquaintance of the German Bettina Gruber.

When Johannes' plan emerges, Gluck knows only one man is capable of stopping him. Margot needs help to get the Goldblatts out.

An attempt to get his crew on side has serious consequences for Johannes. Margot finds a vital ally as Forster reaches St. Remy.

While Swinburne sets sail with a technologically upgraded ship, Ehrenberg makes a serious decision in Kiel. Forster has to create a traitor out of the country.

U-949 meets a Japanese boat to take over heavy cargo - gold. Wilhelm Hoffmann receives an invitation to Lisbon. Forster betrays Klaus Hoffmann under duress what he knows.