Gibbie Gibson (Donald Pleasence) has discovered a World War II-era plane wreck in the mountains of New Zealand. When his discovery gets around town, Gibson, his daughter Sally (Lesley Ann Warren), and his lodger Barney Whitaker (Ken Wahl) find trouble from a group of treasure hunters led by a Mister Theo Brown (George Peppard), who are intent on finding the cache of money they believe is on the wreck.
Inspector Warren of Scotland Yard flies into Jamaica and is taken to the headquarters of the Jamaica Constabulary Force. Exchanging credentials with a Jamaican Inspector, Warren reveals he has come to seek the extradition of a wanted criminal known as "The Professor". Surprised when the Inspector refuses to extradite the Professor, the Jamaican Inspector recounts a story told in flashback.
The story begins with a dramatisation of the Italian manned torpedo raid on Alexandria (1941) in which two British battleships were damaged. The British are worried that this new Italian tactic will give the enemy naval supremacy in the Mediterranean. From Spain, an Italian expert on underwater operations is secretly watching the British base in Gibraltar and planning new attacks.
The film opens with the outbreak of a war in the previously peaceful kingdom of Wredpryd. Count Ferdinand is the inventor of a new submarine who is assigned to command the new ship in battle. The King of Wredpryd orders the Count to sink the "ProPatria" ("for my country"), a civilian ship that is believed to be carrying munitions as well as civilian passengers. In his mind's eye, the Count sees a vision of what would happen if he sent a torpedo crashing into the liner, and he recoils. He refuses to follow his orders, saying he is "obeying orders -- from a Higher Power." Realizing his crew will carry out the orders, the Count fights with the crew and blows up his submarine, sending it to the bottom of the sea.
American fashion model Anne Traymore, swimming off the isle of Majorca, loses a bracelet, which the handsome Manuel del Rio Montoya returns to her. She believes he owns a beautiful yacht called The Swan, but he merely works on it for the wealthy Rene LeClerc.
Un homme est retrouvé assassiné sur la Tamise, l'arme du crime est un fusil-harpon, signature du mystérieux « requin » que Scotland Yard traque sans succès.
Alexandria December, 1941. Two Italian frogmen are captured under suspicion of placing a mine under the HMS Valiant. They are brought onto the ship for questioning.
Captain Hardt (Conrad Veidt), a World War I German U-boat commander, is ordered to lead a mission to attack the British Fleet at Scapa Flow. He sneaks ashore on the Orkney Islands to meet his contact, Frau Tiel (Valerie Hobson). Tiel has taken over the identity of local schoolteacher, Anne Burnett (June Duprez), who had been kidnapped by German agents. Hardt finds himself attracted to her, but Tiel shows no interest. The Germans are aided by a disgraced Royal Navy officer, the former Commander Ashington (Sebastian Shaw) who, according to Tiel, has agreed to aid the Germans after losing his command due to drunkenness, and Tiel implies that she has slept with Ashington to obtain his cooperation.
A US Navy submarine, the USS Corsair, is operating in the North Atlantic, hunting German merchant raiders that are preying on Allied shipping. Its new executive officer, Lt. Ward Stewart (Tyrone Power), has been transferred back into submarines after commanding his own PT boat. At the submarine base in New London, Connecticut, he asks his new captain, Lt. Cmdr. Dewey Connors (Dana Andrews), for a weekend leave to settle his affairs before taking up his new assignment. On a train bound for Washington D.C., Stewart accidentally encounters New London school teacher Jean Hewlett (Anne Baxter) and her students. Despite her initial resistance to his efforts, he charms her and they fall in love.
Somewhere on one of the English-speaking South Sea Islands, at an indefinite present time, with no mention of the fact that it is the middle of World War II, Marge (Gale Sondergaard) is the proprietress of a nightclub/restaurant/gambling hall/brothel called "Isle of Forgotten Sins". In the morning, she knocks on the doors of the individual rooms of her sleeping hostesses, Olga (Betty Amann), Bobbie (Patti McCarty), Mimi (Marian Colby) and Christine (Tala Birell) announcing, "get up, the Southern Queen is here". Riding in a launch from the ship is an inspector from the "Criminal Investigation Bureau" who compares a fingerprint card with a photograph of "Betty Nelson" to one with a photo of a somewhat older "Marge Williams", shows it to the commissioner (unbilled C. Montague Shaw) and tells him, "we'll go there tonight". Marge, surrounded by eight hostesses, is asked by one, "what can we do to please him?" and answers, "he doesn't want to be pleased, he's looking for an excuse to close us up". As the sarong-clad girls leave, one of them, Diane (Rita Quigley) tells Marge, "oh, I suppose it's silly of me but I feel so self-conscious in this costume" and Marge responds, "I know how it is when your modesty's taking a beating, don't let it get you down".
Survival Devices, Inc are an organisation that employ a team of adventurers known as "the Flying Fish" who are adept in sky diving, scuba diving and martial arts. They are engaged to rescue a captured scientist imprisoned on a Caribbean island by a dictator.
Eccentric charter skipper Jim Carnahan (Wilde) and his team of hard-luck dreamers battle sharks, bandits and their own greed to recover sunken treasure off the coast of Honduras.
The American Buckley-class destroyer escort USS Haynes detects and attacks a German U-boat that is on its way to rendezvous with a German merchant raider in the South Atlantic Ocean. Captain Murrell (Robert Mitchum), a former officer in the merchant marine now an active duty lieutenant commander in the Naval Reserve, has recently taken command of the Haynes, even though he is still recovering from injuries incurred in the sinking of his previous ship. Before the U-boat is first spotted, one sailor questions the new captain's fitness and ability. However, as the battle begins, Murrell shows himself to be a match for wily U-boat Kapitän von Stolberg (Curt Jürgens) (portrayed as not being enamoured with the Nazi regime) in a prolonged and deadly battle of wits that tests both men and their crews. Each man grows to respect his opponent.
The World War II U.S. Navy submarine commander P.J. Richardson (Clark Gable) has an obsession with a Japanese destroyer that has sunk three boats in the Bungo Straits, including his previous command. He persuades the Navy Board to give him a new submarine command with the provision that his executive officer, also known as the XO or the "exec", be someone who has just returned from active sea patrol. He single-mindedly trains the crew of his new boat, the USS Nerka, to return to the Bungo Straits and sink the destroyer, captained by a crafty ex-submariner nicknamed Bungo Pete. Richardson's executive officer, Lieutenant Jim Bledsoe (Burt Lancaster), is worried about the safety of his boat and his crew. Bledsoe is also seething with resentment at Richardson and the Navy leadership for denying him command of the Nerka, which he believes should have been his.