Terra Alpha was one of a number of future Earth colonies, ruled by Helen A.
Despite the fact that Helen A. insisted, and punished, people for being unhappy and wearing dark clothes the colony itself was full of dark and dreary streets. Despite Helen A's draconian approach to happiness people were free, provided they could afford it, to travel between colonies. There were scheduled flights, and the colony was monitored by the Galactic Central Bureau.
Any newcomers travelling to the colony had to have a badge issued at customs, something which the Doctor and Ace did not have when they arrived as they never went through customs. They were arrested, but as prisons are depressing they were taken to the waiting zone. Whilst waiting zones had no physical walls there was a line on the floor. Crossing the line would mean the waiting zone attendant would shoot.
Underneath the colony there were a network of disused pipes that used to carry sugar from one of the many sugar mines on the planet. After falling into disuse they were used by the pipe people as a means to get around the colony undetected.
The colony was also home to the Kandy kitchen, where the Kandy Man often experimented with new flavours of sweets, and was the source of the often used Fondant Surprise execution.