Lieutenant Werner is a war correspondent on the German submarine U-96 in October 1941. He is driven by her captain and chief engineer to a raucous French bordello, where he meets some of the crew. Thomsen, another captain, gives a crude drunken speech to celebrate his Ritterkreuz award, in which he mocks Adolf Hitler.
The next morning, U-96 sails out of the harbour of La Rochelle, and Werner is given a tour of the boat. As time passes, he observes ideological differences between the new crew members and the hardened veterans, particularly the captain, who is embittered and cynical about the war. The new men, including Werner, are mocked by the rest of the crew, who share a tight bond. One Nazi officer, 1-WO (the first [watch] officer), is disliked by the others due to his pro-Nazi beliefs and meticulous grooming habits that hogs up the one bathroom that the entire crew has to share. After days of boredom, the crew is excited by another U-boat’s spotting of an enemy convoy, but they are soon spotted by a British destroyer and bombarded with depth charges. They escape with only light damage.
The next three weeks are spent enduring a relentless North Atlantic gale. Morale drops after a series of misfortunes, but the crew is cheered temporarily by a chance encounter with Thomsen’s boat. Shortly after the storm ends, the boat encounters a British convoy and quickly launches three torpedoes, sinking two ships. They are spotted by a destroyer and have to dive below test depth, the submarine’s rated limit. During the ensuing depth-charge attack, the chief machinist, Johann, panics and has to be restrained. The boat sustains heavy damage but is eventually able to safely surface when night falls. A British tanker they torpedoed is still afloat and on fire, so they torpedo it again, only to learn sailors are still aboard. The crew watches in horror as the sailors leap overboard and swim towards them. Neither able nor willing to accommodate prisoners, the captain orders the boat to back away.
The worn-out U-boat crew looks forward to returning home to La Rochelle in time for Christmas, but the ship is ordered to La Spezia, Italy, which means passing through the Strait of Gibraltarβan area heavily defended by the Royal Navy. The U-boat makes a secret night rendezvous at the harbour of Vigo, in neutral although Axis-friendly Spain, with the SS Weser, an interned German merchant ship that clandestinely provides U-boats with fuel, torpedoes, and other supplies. The filthy submariners seem out of place at the opulent dinner prepared for them but are warmly greeted by the Weser’s enthusiastic officers eager to hear their exploits. The captain learns from an envoy of the German consulate that his request for Werner and the chief engineer to be sent back to Germany has been denied.
The crew finishes resupplying and departs for Italy. As they carefully approach the Strait of Gibraltar and are just about to dive, they are suddenly attacked and heavily damaged by a British fighter plane, wounding the navigator, Kriechbaum. The captain orders the boat directly south towards the North African coast at full speed, determined to save his crew even if he loses the boat. British warships begin shelling and they are forced to dive. When attempting to level off, the boat does not respond and continues to sink until, just prior to exceeding its crush depth, it lands on a sea shelf at a depth of 280 metres. The crew works desperately to make numerous repairs before running out of oxygen. After over 16 hours, they are able to surface by blowing their ballast tanks, and limp back towards La Rochelle under cover of darkness.
The crew is exhausted when they finally reach La Rochelle on Christmas Eve. Shortly after Kriechbaum is taken ashore to a waiting ambulance, Royal Air Force planes bomb and strafe the facilities, wounding or killing many of the crew. Ullmann, Johann, the second watch officer, and the Bibelforscher are killed. Frenssen, Bootsmann Lamprecht, and Hinrich are wounded. After the raid, Werner leaves the U-boat bunker in which he had taken shelter and finds the captain, badly injured by shrapnel, watching his U-boat sink in the dock. Just after the boat disappears under the water, the captain collapses and dies. Werner rushes to his body and helplessly surveys the grim scene with tears in his eyes.
