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.
Reynolds plays Caine, a gunrunner who becomes stranded in a small port in the Red Sea. He meets a seductive woman who propositions him to dive into shark-infested waters off the coast for scientific research. However, when Caine realizes the woman and her partner are actually treasure hunters, the action starts to heat up both above and below the water.
Marine scientists prepare to leave their underwater Oceanlab after an extended stay performing oceanographic research. An underwater earthquake interrupts their plans. Dr. Andrews (Walter Pidgeon) enlists experimental sub captain Adrien Blake (Ben Gazzara) to survey the damage and rescue the oceanauts. He brings along Chief Diver "Mack" MacKay (Ernest Borgnine) and Dr. Leah Jansen (Yvette Mimieux), fiancée of one of the scientists. Blake finds the lab has been ripped from its moorings and has tumbled down an unexplored, deep ocean trench, presumably intact. With the lab's reserve air supply dwindling, the team descends into the unexplored trench and finds an incredible ecosystem populated with monstrously over-sized fish. After surviving encounters with unfriendly denizens, they find the lab partially intact, the surviving scientists breathing from scuba tanks, and fending off giant, hungry eels. All but one of the scientists are rescued, and the submarine returns to the surface.
Off the coast of Alaska, oceanographer Emma MacNeil (Deborah Gibson) is studying the migration patterns of whales aboard an experimental submarine she took without permission from her employer. Meanwhile, a military helicopter drops experimental sonar transmitters into the water, causing a pod of whales to go out of control and start ramming a nearby glacier. In the chaos, the helicopter crashes into the glacier, and the combined damage breaks the glacier open, thawing two hibernating, prehistoric creatures. MacNeil narrowly avoids destruction as, unknown to her, a giant shark and octopus are freed. Some time later, a drilling platform off the coast of Japan is attacked by the octopus, which has tentacles large enough to wrap around the entire structure. After returning to Point Dume, California, MacNeil investigates the corpse of a beached whale covered with many bloody wounds. Her employer Dick Richie (Mark Hengst) believes them to be from a tanker propeller, but MacNeil insists they appear to be from a creature. Later, she extracts what appears to be a shark’s tooth from one of the wounds. Elsewhere, the huge shark leaps tens of thousands of feet into the air from the ocean and attacks a commercial aircraft, forcing it to crash into the water.