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. Andrew compares the sleek skyscrapers and rapid modernisation of Shanghai to the colourful street culture and geographic sprawl of Mexico City. He spends a night living in a one-room shack in Dhaka's toughest slum, taking his turn to fetch water, cook and clean; and he rents a friend in the efficient and high-tech, but alienating, city of Tokyo. As he gets under the skin of each unique metropolis, Andrew discovers how the structure of each megacity defines every aspect of its inhabitants' daily lives. And he considers what the megacities of the future can learn from the metropolises of today.

Safety and security are two of the biggest challenges faced by each and every metropolis. Whether earthquake, terrorism, flood or just crime, it's the geology, politics and social makeup of the megacities that make them some of the most profitable and dangerous places to live. Andrew starts in Mexico City, the kidnap capital of the world. The compactness of the megacity often means that the super rich must live closely beside the super poor. Andrew finds out how evasive driving and bulletproof vests are protecting Mexico's super rich and middle classes. In London, he joins a Metropolitan Police riot unit on a practice routine, and hangs out with boy racers in Tokyo. And he meets the canine helpers responsible for saving lives in the event of terrorist attack. Tokyo, Mexico City, Dhaka - the megacities are victims of their shifting geologies. Andrew explores how sea levels, earthquakes and floods are putting some of our most promising cities at risk.

Like human arteries, motorways, roads and train-lines are the lifeblood of any healthy megacity. Whether smoothly flowing or clogged, a city's transport routes affect its inhabitants' quality of life. Andrew Marr finds out how the monstrous megacities stay fed. He also finds out just how hard it is to ride a rickshaw taxi in Dhaka, and discovers how the London tube, once the most ground-breaking transport system in the world, has been usurped by modern transport like Shanghai's 400km/hour magnetic railway. Andrew joins Mexico City's traffic cops in the air, then finds out who is in charge of unblocking Mexico's most filthy canals. He looks into Dhaka's waste management problems, and sees what Britain's fast food obsession is doing to London's sewers.