For the first time in history, more people live in cities than the countryside. Across the globe, we have 21 cities with more than 10 million people, and these numbers are set to increase - busy, noisy, crowded megacities are the future. In a fascinating three-part series, Andrew Marr finds out how these heaving mega-metropolises feed, protect and move their citizens. In the first episode, Andrew looks at how people live in five of the world's biggest megacities: London, one of the world's oldest megacities; Dhaka, the world's fastest-growing megacity; Tokyo, the largest megacity on Earth; Mexico City, one of the most dangerous cities in the world; and Shanghai, arguably the financial capital of the world.