In this first episode, David descends beneath the waves at night in the state-of-the-art Triton submersible, the first of its kind to visit the reef. He meets some of the tiny coral animals that built the reef and helped to turn it into an underwater wonderland. He then takes to the skies to witness the vast scale of their endeavour, a living structure that provides a home for thousands of species. Using cutting-edge technology to generate computer scans of the sea floor, David learns that the Great Barrier Reef we know today is much younger than scientists ever imagined. He meets some of the people that have lived alongside it and who have told stories of its origins for thousands of years. At the beautiful Lizard Island research station, David explores newly discovered research which uncovers a complex community of animals who form remarkable relationships among the coral. But none of this great diversity would be here if it were not for one extraordinary event that happens every year, providing a spectacular finale: the annual coral spawning.

In this second episode, David Attenborough continues his exploration of the Great Barrier Reef, aboard the research vessel Alucia. He discovers the creatures that visit the reef every year, from birds to whales, some travelling thousands of kilometres to get there. Using the latest technology, David dives into the shark-infested waters of Osprey Reef in the Triton submersible. Sixty years after his first visit to Raine Island, he returns to the nesting grounds of the green sea turtle and at Lady Elliot Island marvels at manta ray cleaning stations. New tracking technology allows David to follow the story of visitors like the dwarf minke whales. Stunning satellite imagery and computer animation reveal their journey and David discovers their surprising reasons for returning and why the reef is vital for their survival.

When David Attenborough first visited the Great Barrier Reef in 1957, he considered it the most spectacular place in the natural world and he assumed that it would last forever. Since then the coral has been dying at an unprecedented rate. In this episode he undertakes his most important mission to understand what the next few decades hold for this remarkable community of animals, as well as what is being done to save it. On his journey, David uncovers the remarkable ways in which scientists are trying to preserve the reef and embarks on an ambitious exploration of his own, the deepest dive ever on the reef. But despite the discoveries now being made by scientists and in the reef's furthest reaches, David fears for the future of this incredibly complex and beautiful ecosystem so vital for the world's oceans.