Plasus was the high advisor of Stratos, the capital city of the Federation member-world Ardana in 2269. He followed a policy of apartheid against the Troglytes, the miner-class on the planet Ardana. He believed the Troglytes as inferior, intellectually and physically, and denied them many privileges the Stratos city-dwellers had. When the Troglytes formed a resistance, he was not above torture to crush the resistance. Even his own daughter, Droxine, questioned his methods.
In a visit to the planet to obtain zenite, James T. Kirk became involved in a rebellion. At first, Plasus and his guards rescued Kirk from an attack by the Troglytes. Later, after Kirk objected to Plasus' policies, he asked him to leave the planet and ordered him killed if he returned. Kirk discovered that the Troglytes were being affected by a gas emitted during mining operations. He kidnapped Plasus and held him in a mine with Vanna, the head of the resistance. Plasus was affected by the gas and realized that Kirk was telling the truth. This helped to bring both factions together in order to resolve their differences. (TOS: "The Cloud Minders")