The story is told from the viewpoint of Lt. Werner (Herbert GrÃ¶nemeyer), who has been assigned as a war correspondent on the German submarineÂ U-96 in October 1941. He meets its captain (JÃ¼rgen Prochnow), chief engineer (Klaus Wennemann), and the crew in a raucous French bordello. Thomsen (Otto Sander), another captain, gives a crude drunken speech to celebrate his Ritterkreuz award, in which he openly mocks not only Winston Churchill but implicitly Adolf Hitler as well.
The next morning, they sail out of the harbour of La Rochelle to a cheering crowd and playing band. 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 often mocked by the rest of the crew, who share a tight bond. After days of boredom, the crew is excited by another U-boat's spotting of an enemy convoy, but they soon locate a British destroyer, and are bombarded with depth charges. They narrowly escape with only light damage.
The next three weeks are spent enduring a relentless storm. 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 four torpedoes, sinking two ships. They are spotted by a destroyer and have to dive below the submarine's rated limit. During the ensuing depth-charge attack, the chief mechanic, Johann, panics and has to be restrained. The boat sustains heavy damage, but is eventually able to safely surface in darkness. An enemy tanker remains afloat and on fire, so they torpedo the ship, only to realize that there are still sailors aboard; they watch in horror as the sailors, some on fire, leap overboard and swim towards them. Following orders not to take prisoners, the captain gives the command to back the ship 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 Gibraltarâan 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 officers seem out of place at the opulent dinner prepared for them, but are warmly greeted by 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 Gibraltar and are just about to dive, they are suddenly attacked by a British fighter plane, wounding the navigator. The captain orders the boat directly south towards the African coast at full speed. British ships begin closing in and they are forced to dive; it is later implied that the ships used radar to locate the sub. When attempting to level off, the boat does not respond and continues to sink until, just before being crushed by the pressure, it lands on a sea shelf, at the depth of 280 metres. The crew work desperately to make numerous repairs before running out of oxygen. After over 16 hours, they are able to surface by blowing out their ballast of water, and limp back towards La Rochelle under cover of darkness.
The crew is pale and weary upon reaching La Rochelle on Christmas Eve. Shortly after the wounded navigator is taken ashore to a waiting ambulance, Allied planes bomb and strafe the facilities, wounding or killing many of the crew. Ullmann, Johann and the 2nd Watch Officer are killed. Frenssen, Bootsmann Lamprecht and Hinrich are seriously wounded. After the raid, Werner leaves the U-boat bunker in which he had taken shelter and finds the captain, badly injured by shrapnel and bleeding from the mouth, watching the U-boat sink at the dock. Just after the boat disappears under the water, the captain collapses and dies. Werner runs to his body, recoils, and quickly glances around at the destruction, his face frozen with distress. He then looks down at the captain's body, with tears in his eyes.
When does the crew reach La Rochelle?
Christmas Eve