Sprawling, noisy, and often downright dangerous, the world's megacities buzz with energy and innovation. Today, there are more than 20 metropolises with over 10 million inhabitants. Some of them are sleek, modern, and high-tech; others filled with slums and blighted by poverty; all of them fragile places constantly on the edge. Together they are the greatest shift in human civilization since the birth of agriculture... and man's biggest and most dangerous social experiment yet. BBC journalist Andrew Marr visits five megacities: one of the oldest (London, United Kingdom), the fastest growing (Dhaka, Bangladesh), the largest (Tokyo, Japan), among the most hazardous (Mexico City, Mexico), and one that considers itself the new world capital (Shanghai, China). Marr discovers how the cities' structures define every aspect of their inhabitants' daily lives, and how these great urban centers represent the human future of the planet.