In 1405 a 300-strong fleet of giant treasure shps set out from the Chinese port of Nanjing on the first of seven epic voyages to western Asia, Africa and Arabia. China had embarked on a golden period of exploration and trade and became the most advanced sea-faring nation in the World. The commander of this fleet was a scholar a diplomat, a warrior - and a eunuch. His name was Admiral Zheng He. Six hundred years later, as the country opens its doors again to the world, Tim Luard visits China to tell the story of its greatest explorer, producer Bryn Jones