Set in London during the 1850s, this episode focuses on the construction of the London sewerage system, built to replace the antiquated medieval system that was overworked and inadequate for the needs of the-then largest metropolis in the world, causing epidemics of disease and a permanent foul stench to fill the air. The episode follows the efforts and work of Joseph Bazalgette, the brilliant engineer who designed the influential and modern sewer system that would purify the city, transform the streets above and would result in the end of the epidemics of cholera and typhoid that had ravaged the population - although not for the reasons that he initially thought.