The Tellun system was an inhabited planetary system. The two planets of this system, Elas and Troyius, were the homeworlds of the Elasians and Troyians respectively. This system was located in the Federation-Klingon border.
In 2268, both civilizations possessed the capability of mutural destruction. In that year, both the Federation and the Klingon Empire considered the Tellun system to reside within their respective borders. The Federation sought an end to the internal conflict and served as a mediator between Elasians and Troyians, while Klingons tried to subvert the peace efforts.
The USS Enterprise, a Federation starship, was assigned a secret diplomatic mission. Their mission: to facilitate the marriage of Elaan, the Dohlman of Elas, to the ruler of Troyius. This mission was interrupted by sabotage and an indecisive ship-to-ship engagement with a Klingon D7 class battle cruiser. During the course of battle, the Klingons' true interest in the Tellun system, access to Elasian dilithium deposits, was discovered. Regardless of these obstacles, the mission was a success. (TOS: "Elaan of Troyius")