David Attenborough is in the Swiss Jura Mountains to discover the secrets of a giant. Beneath his feet lies a vast network of tunnels and chambers, home to a huge empire of ants. It is believed to be one of the largest animal societies in the world, where over a billion ants from rival colonies live in peace. Their harmonious existence breaks many of the rules for both ants and evolution, and raises some important questions. Through winter, spring and into summer, David turns detective to find the answers.