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After sinking a merchant ship from an Allied convoy, German U-boat U-571 has her engines badly damaged by depth charges from a British destroyer. U-571's skipper Kapitänleutnant Gunther Wassner makes a distress call that is intercepted by American intelligence, so the US Navy has submarine S-33 modified to resemble a German resupply U-boat to steal the Enigma coding device and sink the U-571. As the crew of S-33 receive their assignment, the submarine's executive officer Lieutenant Tyler is unhappy about his promotion being blocked by commanding officer Lieutenant Commander Dahlgren.
During a storm, S-33's boarding party surprises and overwhelms the crew of U-571. After securing U-571, the American S-33 is torpedoed by the German resupply sub that was sent to aid U-571. Lieutenant Commander Dahlgren is blown off the deck of S-33 and seriously wounded, while struggling to stay afloat he refuses rescue and orders the boarding party on the captured U-boat to submerge. Lieutenant Tyler takes command of U-571 and dives below the surface where they engage and sink the resupply sub in an underwater battle.
After making repairs and restoring power Tyler decides to route the disabled submarine to Land's End in Cornwall, England. They are spotted by a German reconnaissance plane which is unaware that U-571 has been commandeered by Americans; a nearby German destroyer sends over a small contingent but right before boarders arrive, Tyler gives orders to fire a shot from the deck gun, destroying the German destroyer's radio room, and preventing it from reporting the capture of a German sub and its Enigma machine and code books. The sub dives underneath the German destroyer, which then begins to drop depth charges to sink U-571. U-571's original master Kapitänleutnant Wassner escapes captivity and kills one of Tyler's crew but he is subdued before he can sabotage the boat's engines.
Tyler attempts to trick the destroyer into stopping its attack by ejecting debris and a corpse out of a torpedo tube, faking their own destruction, however the German destroyer continues dropping depth charges. The crew then realizes that Kapitänleutnant Wassner, despite being shackled, is using Morse Code tapping to signal to the destroyer that the submarine was captured and knock him out. U-571, hiding at below 600 feet (180 m), is damaged by the high water pressure. Control of the main ballast tanks is lost and the ship ascends uncontrollably. Tyler orders crewman Trigger to submerse himself in the bilge underwater to repressurize the torpedo tubes.
Trigger uses an air hose to enter the flooded compartment. He closes the air valve to the torpedo tubes, but a second leak and broken valve are found, which Trigger can't reach. U-571 surfaces heavily damaged and begins to flood, unable to fire its last torpedo from its stern tubes. The destroyer gives chase and fires upon U-571 with its main guns; the first hit causes pipes to collapse, pinning Trigger's leg, after he has left the air hose behind. Unable to turn back, he reaches for the valve and closes it before he dies. The second the pressure is available, Tyler orders Tank to fire the final torpedo. The German destroyer is unable to take evasive action and is sunk. As the crew sigh in relief, Tank reports Trigger's death. U-571 has taken severe damage and will not remain afloat for long. The crew abandons the submarine with the Enigma in their possession. The crew watch U-571 as she slips beneath the waves. Floating aboard an inflatable lifeboat, they are eventually rescued by a US Navy PBY Catalina flying boat.
psychedelic, romantic