Private Thurston was a UNIT soldier serving under Brigadier Lethbridge-Stewart. In the 1970s, he was one of the UNIT soldiers stationed at Tulloch to investigate the destruction of several oil rigs in the North Sea.
He and several other soldiers later went to the World Energy Conference to try and track down the Zygon warlord Broton. After the Fourth Doctor discovered Broton, Thurston and the Brigaider shot and killed the Zygon. (TV: Terror of the Zygons)