In the fourth and final episode, Mary tackles the biggest puzzle of all: why, and how, did the Roman Empire fall?
Surveying the massive walls and fortifications of Britain and Germany, she discovers an empire under pressure, struggling to control its borders.
Mary seeks to redefine our understanding of the so-called 'Barbarian Invasions', but also shows that the Roman Empire was facing even greater challenges from within.
Maverick emperors upset all the assumptions of right-thinking Romans, while the traditional religion and beliefs of the Roman state came head to head with the absolute conviction of Jews and Christians. Ultimately, Mary asks whether the Roman Empire was transformed rather than destroyed, and indeed lives on in the world we still see all around us - in our institutions and infrastructure, in the aspirations, methodology and symbolism of many empires since.