Our planet currently sustains some 21 megacities, vast metropolises that are home to populations of at least ten million each. Andrew Marr explores five of them: Dhaka, home to 13 million, built on a fertile flood plain, subject to flooding and disease; Tokyo, with a population of 33 million, built on three of the most unstable fault lines in the world; Mexico City, sustaining 20 million, built on a lake and drowning under a tidal wave of crime; London, home to 13 million, with a food footprint more than 125 x its size; and Shanghai, with its 23 million people - including 7,000 billionaires. What draws people magnetically to the megacities? How much - or how little - planning, construction and care does it keep to sustain them; and are they, ultimately, sustainable? From staying in a corridor flat in Tokyo to sleeping in a slum shack in Dhaka; from the heights of a Shanghai penthouse to the depths of a London sewer, Andrew Marr explores the man made arteries and beating hearts of these - the megacities.