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.

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.

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.