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.
On a volcanic island near the kingdom of Hetvia rules Count Dakkar, a benevolent leader and scientist who has eliminated class distinction among the island's inhabitants. Dakkar, his daughter Sonia and her fiance, engineer Nicolai Roget have designed a submarine which Roget pilots on its initial voyage just before the island is overrun by Baron Falon, despotic ruler of Hetvia. Falon sets out after Roget in a second submarine and the two craft, diving to the ocean's floor, discover a strange land populated by dragons, giant squid and an eerie undiscovered humanoid race.
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.
One day in New Guinea, two men, "Jumbo" Johnson and seaplane pilot Nick Brandon, are discussing a possible gold heist. They have a drink with Katherine Shelley, an attractive widow who's obviously drinking away her sorrows.
The film begins shortly after the outbreak of World War II when Günther Prien reports to the Befehlshaber der U-Boote (BdU—supreme commander of the U-boat Arm) Karl Dönitz. Dönitz orders Prien as commander of U-47 to penetrate the Royal Navy's primary base at Scapa Flow to inflict as much damage as possible. Prien accomplishes his mission and receives a hero's welcome on his return.
The new, state of the art nuclear submarine Seaview is on diving trials in the Arctic Ocean. The Seaview is designed and built by scientist and engineering genius Admiral Harriman Nelson (USN-Ret) (Walter Pidgeon). Captain Lee Crane (Robert Sterling) is the Seaview's Commanding Officer. One of the on-board observers is Dr. Susan Hiller (Joan Fontaine), studying crew-related stress. The mission includes being out of radio contact for 96 hours while under the Arctic ice cap, but the ice begins to crack and melt, with boulder-size pieces crashing into the ocean around the submarine. Surfacing, they discover fire burning in the sky. After the rescue of a scientist and his dog at Ice Floe Delta, Miguel Alvarez (Michael Ansara), the sub receives radio contact from Mission Director Inspector Bergan at the Bureau of Marine Exploration. He advises that a meteor shower pierced the Van Allen radiation belt causing it to catch fire, resulting in a world-threatening increase in heat all across the Earth. Nelson's on-board friend and scientist, retired Commodore Lucius Emery (Peter Lorre) concurs that it is possible. Bergan informs Nelson that the President wants him at a UN Emergency Scientific Meeting as soon as possible.
The film begins with a tough Greek Lieutenant (J. Carrol Naish) announcing that the United States Marine Corps is seeking volunteers for a hazardous mission and special unit. Sergeant "Transport" Anderof (Sam Levene) meets the commander of the unit, Lieutenant Colonel Thorwald (Randolph Scott) who he has served with in the China Marines. Thorwald explains that he left the Corps to serve with the Chinese guerrillas fighting the Japanese during the Second Sino-Japanese War to learn their methods and has decided to form a unit using the qualities of Gung Ho or "work together".
Gregory Mason and Andy Callahan are partners in a salvage business in Manila. Callahan is murdered after some drunken talk about sunken treasure; Mason is suspected by Lt Zuenon of the local police because he is in love with Callahan's wife, Stella.
Seventeen-year-old Joshua "Josh" McGuire (Rhys Wakefield), expedition bank-roller Carl Hurley (Ioan Gruffudd) and his girlfriend, Victoria "Vic" Elaine (Alice Parkinson), travel to the Esa'ala Cave, an underwater cave exploration site in Papua New Guinea. Josh's father, Frank (Richard Roxburgh), a master diver, has already established a forward base camp at a lower level inside the cave, where the team has been exploring for weeks. As Josh begins to advertise his disdain for his father and his opinions about cave exploration, the team below prepares to dive into an unexplored area of the system.
At Aquatica, a remote former submarine refueling facility converted into a laboratory, a team of scientists is searching for a cure for Alzheimer's disease. Fluids from the brain tissue of three Mako sharks are being harvested as a cure for Alzheimer's. Unknown to the other scientists, Dr. Susan McAlester (Saffron Burrows) has violated the code of ethics and has genetically engineered the sharks to increase their brain size, but this has the side effect of making the sharks smarter and more dangerous.
Three men (Dr. Ken Tashiro, Dr. Jules Masson, and Perry Lawton) are trapped in a bathysphere due to seismic activity. They are rescued by the crew of the supersubmarine Alpha, captained by Craig Mackenzie (Cotten), who they learn is over 200 years old (and that the Alpha was launched in the early 19th century). Mackenzie takes them to Latitude Zero to deal with the serious injuries of Dr Masson. While returning to Latitude Zero, they are attacked by a rival supersubmarine, the Black Shark, captained by Kuroi, who works for a rival of Mackenzie, Malic, who is also over 200 years old and has a base at Blood Rock.
Demi (Harris), is a veteran Soviet Navy captain finishing up a career that failed to live up to the legacy of his legendary father. He is given an assignment by Markov (Henriksen) to lead a top secret mission and given the command of his old ship. Due to mistakes he made during his service, involving the deaths of many of his past crew in a fire, this will be his last assignment and his only opportunity to command any ship at all. Demi interrupts a party involving his crew, including Alex Kozlov (Fichtner), his up-and-coming executive officer headed for great success. However, as Demi leaves his home to lead Alex and his crew of on the secret mission, the presence of KGB agent Bruni (Duchovny), and ominous portents, begin to alter the objectives of the mission, causing Demi and Kozlov to become concerned. Furthermore, Markov commits suicide as the submarine departs.