Season 4

April 1940. When Hitler dispatches the U-Boat arm to protect troops invading Norway, a German submarine rushes towards the deep fjords leading to Narvik, the scene of two major deadly naval battles. While multiple encounters with British Royal Navy ships provide numerous targets, including the battleship HMS Warpsite, torpedo failures threaten their mission.

The career of a bright, young U-Boat officer begins with promise when he is awarded command of his own submarine. But during their first two war patrols, clashes with the Executive Officer and Chief Engineer over Nazi ideology cause tension and outright conflict. The officers' complaints lead to Commander's court martial and the threat of execution.

A German U-Boat, desperately low on supplies and with the crew suffering from scurvy, must be re-fueled from a supply submarine to make it back to port. With Allied Hunter Killer groups tracking their every move, the U-Boat makes a desperate run for survival. When a destroyer closes in to ram, the enemy crew uses the opportunity to board and attack the American ship.

After an enemy attack forces an American submarine into a disastrous dive, the crew struggles to regain control while battling a deadly chemical fire that threatens their limited air supply.

October 1943. After their first war patrol ends in conflict and disappointment, a new submarine Commander must overcome dissent despite his unconventional choices for key roles. Under his leadership, the crew works together to build trust even as they travel to target enemy shipping in dangerous Japanese home waters.

In February 1945, an American submarine Commander receives an order to hunt Japanese submarines assigned to secretly evacuating VIPs from the Philippines. Their job to send fellow submariners to the ocean floor sparks a battle of tactics, stealth and risk that would set a record for the U.S. Navy in World War II.