Divers Martha Holmes and Mike de Gruy head for Australia, for an event described as "the greatest sex show on earth" - the mass spawning of corals. While waiting for the curtain to rise, they are surrounded by circling grey reef sharks. In a less sinister encounter they face a posse of giant groupers. Up on the beach, the divers watch entranced as turtles lumber ashore to lay eggs. Then, in a fitting climax on the chosen moonlit night, the corals all along this spectacular reef release their eggs and sperm in an underwater extravaganza to rival any firework display.