By 2050, two thirds of the earth's inhabitants will live in cities. This seismic urban shift in the world's exploding population is one of the biggest challenges facing humanity. Andrew Marr 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. Megacities increasingly have more inhabitants than small countries, with far-reaching consequences: a growing wealth gap, widespread social unrest and increased mortality. This landmark docu-series celebrates human ingenuity on an epic scale, revealing how these metropolises tick...and the enormous challenges they face. In a journey of discovery around the world to reveal the anatomy of some of the world's most incredible cities, audiences meet the engineers and technocrats who are in control, along with the ordinary people who must live with their decisions. Most megacities lie at sea level, in seismic zones or in hurricane areas. Can humans cope with the impact of disasters of unprecedented magnitude?