Medok was an inhabitant of a humancolony secretly ruled by Macra. He discovered the existence of the Macra, but was believed by the others to be insane. He succeeded in alerting the Second Doctor to the threat. When he was captured, he was sent to the gas mines to work. He was killed by a Macra when he tried to escape. (TV: The Macra Terror)
In Ian Stuart Black's novelisation of the story, Medok survives his encounter with the Macra in the mine, and emerges just in time to witness the Doctor and his companions departing in the TARDIS — deciding there and then never to breathe a word of what he has just witnessed.