George Trenchard was a human who was in charge of the prison on Fortress Island where the Master was incarcerated. He told the Master that he used to be the governor of a colony once.
Shortly after the Master arrived, he persuaded Trenchard that "enemy agents" were working against him and the country. These "enemy agents" were also responsible for ships recently sunk in the area. Trenchard willingly helped the Master, going so far as to distract John Hart with talk of golf, while the Master stole various pieces of equipment.
Following the Master revealing his contact with the Sea Devils and a subsequent Sea Devil attack, he was killed after he shot a Sea Devil. (TV: The Sea Devils)