Garthak was the leader of the resistance fighting against the High Priest on Florana. He and the other resistance members helped rescue Ella and Tegan Jovanka from a train taking them to a work camp. He later took part in an attack to allow Tegan and Ella to enter the city. After the population of the city began to attack the military HQ, Garthak helped to defend it.
Garthak later took a fighter jet, planning to crash it into the cathedral to stop Thane from activating the Cleansing Fire. Despite the Fifth Doctor's protests, Garthak crashed the jet into the cathedral, killing himself, but stopping the Cleansing Fire. (AUDIO: The Elite)