There was no lighting in the undercity save for candles, nor were conditions favourable; much of the surface was covered in filth.
Mandrel was the leader of the outlaws living in the undercity. They were forced into the city whenever they wanted food as they had no local sources. Many of those aboveground had heard of the undercity and those that lived there, but knew few details.
After he was unable to pay his father's death tax, Cordo headed for the undercity. The Fourth Doctor and Leela joined him. Upon arrival, Mandrel told them they could stay if the Doctor delivered a consumcard to the Consum Bank on subway 37. Mandrel threatened him, saying he had to return before a set time or Leela would be killed. Cordo went with the Doctor to show him the way.
Cordo made his way back to the undercity after some time and reported the Doctor captured. At this news, Leela and Cordo left to rescue him. The Doctor returned on his own some time later, having been freed. Mandrel did not believe this and readied a hot iron to torture him. A friend of the Doctor's, Bisham, used a balcony above the undercity's main level to threaten him into surrender. Mandrel agreed to help the Doctor take over Main Control while the rest of the outlaws stirred up rebellion among the workers. This left the undercity abandoned. (TV: The Sun Makers)