The Tesh were a group of humans descended from the technician survivors of the Mordee expedition. They worshiped Xoanon, whom they believed to be a god. Xoanon was actually a supercomputer that had created the Tesh and the rival Sevateem as artificial culture groups, in a bid to create superhumans. Xoanon listed the characteristics of the Tesh as "self-denial, control" and "telepathy".
Xoanon used conflict between the Tesh and the Sevateem to speed up the process of separating the two tribes into distinct groups. The Tesh regarded their counterparts as savages. They lived in the ship that the humans originally landed in and had developed psychic powers to the point where they were able to disable humans and Time Lords with their minds.
Jabel was the leader of the Tesh when the Fourth Doctor encountered them. He used his psychic powers to disable Leela and had her submitted for particle analysis, hoping this would provide the secret as to how the Sevateem were able to bypass the barriers created on the planet by Xoanon, even though the Doctor tried to tell Jabel that Xoanon was the one responsible for opening the barriers. The Doctor too was submitted for analysis when Jabel decided he was not the Lord of Time.
The Doctor freed himself and Leela, prompting the Tesh to search the ship for them. Leela killed several with small arms fire. After the Doctor removed his own personality from the mind of Xoanon and the computer was restored to sanity, the tribes of Tesh and Sevateem set about trying to co-operate and the Doctor left them to their own devices. (TV: The Face of Evil)