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.