In the final episode of his series tracing the story of the most famous trade route in history, Dr Sam Willis continues his journey west in Iran. The first BBC documentary team to be granted entry for nearly a decade, Sam begins in the legendary city of Persepolis - heart of the first Persian Empire. Following an ancient caravan route through Persia's deserts, he visits a Zoroastrian temple where a holy fire has burned for 1,500 years, and Esfahan, one of the Silk Road's architectural jewels and rival to Sam's next destination - Istanbul. In the ancient capital of Byzantium Sam discovers how the eastern Roman Empire was ruled through silk and how Venetian merchants cashed in on the wealth and trade it generated. Sam's last stop takes him full circle to Venice. Visiting Marco Polo's house, Sam reminds us how the great traveler's book was one of the first to link East to West and how the ideas and products that trickled down the Silk Road not only helped to trigger the Renaissance, but set Europe on a path of unstoppable change.