Captain Cousteau and his ship the Calypso visit the Red Sea, the Indian Ocean and the Gulf of Aden in order to study shark behaviour and to test methods of protecting shipwreck and air crash victims from shark attacks. Ichthyologist Dr. Eugenia Clark joins the Calypso crew to study shark behavior and test shark repellents.

The team travels to the coral reefs of the Indian Ocean, an area teeming with animal and plant life, and where numemous complex relationships between different species are played out on a daily basis. Cousteau and his men find a well-ordered universe housing a highly complex community on the coral reef.

The island ot Europa, in the Mozambique Channel between Africa and Madagascar, is the foremost breeding ground of the mysterious sea turtle. The crew's cameras follow the turtles' mating rituals, the females' laboured journey up the beach to lay their eggs, and the fight for survival of the newly-hatched babies. The Calypso crew films the undersea mating dance and subsequent egg-laying of the sea turtle.

The Calypso divers study finback, sperm and killer whales, monitoring their migratory habits by planting small tags in their skin. Captain Cousteau also recounts the romance and legend of the great whales. Cousteau and his crew implant tracking devices into whales off the coasts of Madagascar and California.

Captain Cousteau and the crew of the Calypso adopt a pair of fur seals off the Cape of Good Hope. During an Atlantic crossing the men begin to establish a relationship of trust and friendship with the mammals. Cousteau and his crew take two wild seal pups aboard for study.