In the waters off the coast of southern California, divers from the Scripps Institute of Oceanography prepare to study ocean life and conditions and to collect specimens. It is imperative that the divers pay close attention to their gear, for even though their aqualungs provide them with 2,000 pounds of compressed air, if the air flow is not regulated precisely, the pressure could kill them. Also vital is the synchronisation of the divers' watches, as they have only half an hour of air, ten minutes of which must be preserved to rise slowly to the surface to avoid the bends. Some of the divers carry motion picture cameras, while others carry spear guns to protect against sharks.

The first group of men dive into the ocean and immediately spot many forms of sea life in the kelp forest, including a giant tortoise, a stingray, a sand shark and a full-grown shark, from which they keep a discreet distance. One of the divers bayonets a huge sea bass, while others watch a group of sea lions cavorting nearby. A small but tenacious Garibaldi fish, enraged by the presence of a starfish near its nest, catches the attention of the divers, while the second group of men investigate a submarine canyon twenty-five miles from San Diego harbour.

The depth of the canyon, which has never been explored before, is unknown, and the divers, while photographing at a depth of one hundred feet with natural light, also take geological samples. Meanwhile, the first group of divers are alarmed when the shark comes dangerously nearby, and so one of the men shoots it with his spear gun. The lead from the gun becomes entwined around the diver's leg, however, and he is trailed helplessly behind the furiously swimming shark until the lead breaks and he is freed. Another diver rescues the injured man, sharing his oxygen supply with him until they reach the surface.

The film then discusses the plight of the giant sea elephants, which once roamed the Pacific in large numbers but have now, through over-hunting by man, been reduced to approximately 2,000. The majestic sea elephants, ungainly on land but graceful in the ocean, are now protected, and are returning to a normal population. Near Guadalupe Island, a cameraman is approached by a curious sea elephant pup, who is friendly but, not knowing his own strength, easily pushes the diver off balance. The divers also observe the great ocean sunfish, which, at 1,000 pounds, feeds on jellyfish. The jellyfish's unique form of reproduction, through which coral reefs are built, is discussed, as is the animal's deadly sting.

Near Santa Catalina Island, divers encounter a tasty grouper and catch him for their dinner, then observe other animals such as a spotted skate, a hermit crab, a porcupine fish and a huge lobster, which is also added to the dinner pot. One diver comes across an octopus, which, contrary to popular myth, is more frightened of the man than the man is of him. A large school of hammerhead sharks is spotted and filmed from a distance, as is an enormous basking shark, which swims with its huge mouth open to catch plankton. Close to Mexico, divers encounter the giant manta ray, which can grow up to 3,000 pounds and whose young measure four feet across at birth.

Hundreds of rays swim by in an eerie procession. Later, the divers travel to the clear Atlantic waters near the Bahama Islands. There, the explorers encounter a moray eel, one of the most dangerous fish in the ocean, and watch as it is eaten by a shark.

One of the divers captures another eel, then watches as other sea life such as the lovely parrot fish, French Angelfish and Queen Angelfish swim by. The men also enjoy the antics of the gobi fish, which digs its home in the sandy ocean bottom and occasionally pops up to spy on its neighbours, such as a wandering conch. Much to the divers' surprise and delight, they are "adopted" by a pair of groupers, which follow them with dog-like devotion throughout their two years of exploration. One, a sixty-pound striped grouper, is named Sapphire, while the other, an eighty-pound yellow-fin, is named Uncle Remus. The fish locate the divers whenever they appear in the area and follow them about, pestering the cameramen and eating from the hands of the men who catch smaller fish for them. While tending to their "pets," the scientists observe the lightning-fast barracuda, the tiny but equally fast needle fish, and the squid, with its own method of jet-propulsion. Emboldened by his success in feeding Sapphire and Remus, one diver even places a fish on a spear to feed a large shark, and is relieved when the hungry creature eats only the fish and not him. The diver then watches two male stingrays fight over a female, with the injured loser being eaten alive by predators. Later, the divers pet Sapphire and Remus and reluctantly say goodbye, but are certain that the devoted groupers will be waiting for their next annual visit.