The Doradans were enslaved by the Masters of Dorada to work in underground factories, supplying the necessities and luxuries the Masters demanded. They lived their whole lives down there, being given electric shocks if they stopped working, pleasant dreams if they went to sleep on time, and horrific nightmares if they didn't.
Some Doradans took refuge in the factories' ventilation shafts; those who were born there were immune to the Masters' control. They developed helmets which would block out the Masters' control, but could not use them themselves and due to scarcity of the required materials were only able to make three of them. When the Second Doctor, Ben Jackson and Polly Wright arrived on the planet and were enslaved as well, a rebel Doradan used the helmets to free them, then asked them to kill the Masters. When they refused, he and another Doradan convinced them to do so by approaching the Masters, who destroyed the intelligence of one and killed the other. Shaken, the Doctor and his companions did what they asked, freeing the Doradans. (PROSE: The Dream Masters)