The story is told from the viewpoint of Lt. Werner (Herbert Grönemeyer), who has been assigned as a war correspondent on the German submarine U-96 in October 1941. He meets its captain (Jürgen Prochnow), chief engineer (Klaus Wennemann), and the crew in a French nightclub. 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.
In 1943, the Axis powers plan an assault on the island of Keros — where 2,000 British soldiers are marooned — to bully neutral Turkey into joining them with a display of their military strength. Rescue by the Royal Navy is prevented by two massive radar-directed superguns on the nearby island of Navarone. When aerial bombing efforts fail, Allied Intelligence gathers a team of commandos to infiltrate Navarone and destroy the guns. Led by Major Roy Franklin (Anthony Quayle), the team is composed of Captain Keith Mallory (Gregory Peck), a renowned spy and mountaineer; Colonel Andrea Stavrou (Anthony Quinn) from the defeated Greek army; Franklin's best friend Corporal Miller (David Niven), an explosives expert and former chemistry teacher; Greco-American Spyros Pappadimos (James Darren), a native of Navarone; and "Butcher" Brown (Stanley Baker), an engineer and expert knife fighter.
The story, loosely based on actual events, takes place in March 1943, when the Second World War was at its height. The cryptanalysts at Bletchley Park, Buckinghamshire, have a problem: the Nazi U-boats have changed one of their code reference books used for Enigma machine ciphers, leading to a blackout in the flow of vital naval signals intelligence. The British cryptanalysts have cracked the "Shark" cipher once before, and they need to do it again in order to keep track of U-boat locations.
Steve MacKendrick (Alan Ladd), nicknamed "Canada" because that is where he claims he is from, volunteers in 1940 for the British Army's paratroop school. He obviously has a good deal more background and leadership skills than he lets on. Canada tries to become better acquainted with a pretty parachute rigger named Penny Gardner (Susan Stephen). She is initially put off by his attitude, but they eventually start dating. Both Penny and his new commander, Major Snow (Leo Genn), see potential (and a mystery that does not add up) in him, despite his strong efforts to avoid assuming any responsibility. Canada turns down Snow's offer to send him to officer school.
Captain Karl Ehrlich (John Wayne) is the master of the elderly German steam freighter Ergenstrasse, in port at Sydney, Australia on the eve of the Second World War. Ehrlich is depicted as a patriot, a former career naval officer who lost his rank and position after falling out of favour with the current regime and refusing to support the Nazi Party. As his ship prepares for sea (or to be interned if war is declared) he meets with an old friend, British Commander Jeff Napier (David Farrar) and his fiancée Elsa Keller (Lana Turner).
Early in the Second World War, U-37, a German U-boat, makes its way to Canadian waters and participates in the Battle of the St. Lawrence. It succeeds in evading an RCAF patrol and moves north. While a raiding party of six Nazi sailors is put ashore in an attempt to obtain supplies, the U-boat is sunk in Hudson Bay. The six attempt to evade capture by traveling across Canada to the still-neutral United States.
Lieutenant Commander Charlie Madison (James Garner), United States Naval Reserve, is a cynical and highly efficient adjutant to Rear Admiral William Jessup (Melvyn Douglas) in 1944 London. Madison's job as a dog robber is to keep his boss and other high-ranking officers supplied with luxury goods and amiable Englishwomen. He falls in love with a driver from the motor pool, Emily Barham (Julie Andrews), who has lost her husband, brother, and father in the war. Madison's pleasure-seeking "American" lifestyle amid wartime rationing both fascinates and disgusts Emily, but she does not want to lose another loved one to war and finds the "practicing coward" Madison irresistible.
In 1939, Nazi Germany's largest and most powerful battleship, Bismarck, is launched in a ceremony at Hamburg with Adolf Hitler attending. The launching of the hull is seen as the beginning of a new era of German sea power.
An American oil tanker mastered by Capt. Steve Jarvis (Raymond Massey) is sunk in the north Atlantic Ocean by a German U-boat. He and the first officer, his friend Joe Rossi (Humphrey Bogart), make it to a lifeboat along with other crewmen. When the U-boat crew starts filming their plight they respond with rude gestures and are rammed. The men swim to a raft and are rescued after 11 days adrift.
The film opens with the narration: "This is the story of a ship" and the images of shipbuilding in a British dockyard. The action then moves forward in time showing the ship, HMS Torrin, engaging German transports in a night-time engagement during the Battle of Crete in 1941. However, when dawn breaks, the destroyer comes under aerial attack from German bombers.
Loosely based on actual events during World War II, the film depicts real life German Captain Bernhard Rogge commanding the navy raider Atlantis, which from May 1940 to November 1941 sank 22 Allied merchant ships. The story alternates between scenes at the Admiralty and scenes at sea, particularly showing Captain Rogge's humanity and chivalrous conduct of his military engagements. Rogge was one of the few German flag rank officers who was not arrested by the Allies after the war, due to his conduct as a military officer. After eighteen months of successful raids, Atlantis was sunk on 22 November 1941 by the British cruiser Devonshire.
Several British and U.S. civilians, service members and merchant marines are stuck in a lifeboat in the North Atlantic after their ship and a U-boat sink each other in combat. Willi (Walter Slezak), a German survivor, is pulled aboard and denies being the U-boat's captain. During an animated debate, Kovac (John Hodiak) demands the German be thrown out to drown. However, the others object, with Stanley (Hume Cronyn), wealthy industrialist Rittenhouse (Henry Hull) and columnist Connie Porter (Tallulah Bankhead), who speaks German, succeeding in arguing that he be allowed to stay. Porter, initially alone in the boat, had managed to bring her luggage with her, and her primary concern at first is a run in her stocking. She is thrilled at having filmed the battle between the two vessels, but her movie camera is the first in a series of her possessions to be lost overboard in a succession of incidents.
The USS Tiger Shark is a U.S. Navy submarine on patrol in the Atlantic Ocean during World War II in August 1943. She receives orders to pick up survivors spotted adrift by a British PBY Catalina patrol plane. She retrieves three survivors – the British nurse Claire Paige (Olivia Williams) and two men, one of them wounded – from the British hospital ship Fort James, which had been sunk two days earlier; one of the survivors blames the sinking on a German U-boat that he briefly saw on the surface just before the Fort James suffered a torpedo hit. As they pick up the survivors, the crew of the Tiger Shark spots a German warship bearing down on them. The submarine has several encounters with the German warship and suffers damage from depth charges in the process. Later, the commanding officer of the Tiger Shark, Lieutenant Brice (Bruce Greenwood), discovers that the wounded survivor is actually a German prisoner-of-war, Bernard Schillings (Jonathan Hartman). Brice confronts him because he thinks Schillings has been making noises to betray the Tiger Shark 's position to the German warship. Brice shoots Schillings dead when the German panics and grabs a scalpel to defend himself.
The film tells the story of the crew aboard a British tramp steamer named the SS Glencairn on the long voyage home from the West Indies to Baltimore and then to England. The crew is a motley, fun-loving, hard-drinking lot. Among them is their consensus leader, a middle-aged Irishman named Driscoll ("Drisk") (Thomas Mitchell), a young Swedish ex-farmer Ole Olsen (John Wayne), a spiteful steward nicknamed Cocky (Barry Fitzgerald), a brooding Lord Jim-like Englishman Smitty (Ian Hunter), and a burly, thoroughly dependable bruiser Davis (Joseph Sawyer), among others. The film opens on a sultry night in a port in the West Indies where the crew have been confined to their ship by order of the captain, yet they yearn as ever for an opportunity to drink and have fun with the ladies. Drisk has arranged to import a boat-load of local ladies, who along with baskets of fruit have agreed to smuggle bottles of rum on board where, with the acquiescence of the captain, the crew carouse until a minor drunken brawl breaks out and the ladies are ordered off the ship and denied any of their promised compensation. The next day the ship sails to pick up its cargo for its return trip to England. When the crew discovers that the cargo is high explosives, they at first rebel and grumble among themselves that they won't crew the ship if it is carrying such a cargo. But they are easily cowed into submission by the captain and the ship sails, crossing the Atlantic and passing through what they all know is the war zone and potential disaster.