Balkos III was a planet, a class M planet that was homeworld to the Balkosi, a primitive humanoid civilization that was just entering its own stone age in the late 2260s. The third planet in the Balkos star system, Balkos III had been visited by the Brassica sometime prior to that decade, and served as one of their testing sites designed to investigate other galactic civilizations.
Preliminary United Federation of Planets studies of the world suggested that the native inhabitants were relatively new to the planet and should not have reached their current level of societal development so quickly. In response, the USS Demeter was dispatched to investigate. However, its initial scans trigged a massive sensor probe from the planet, indicating the presence of large and sophisticated machinery. Fearful of activating an ancient planetary defense system, the Demeter's captain, Commander John Gellman, contacted the USS Enterprise and requested assistance.
A landing party consisting of James T. Kirk, Spock, and Leonard McCoy discovered a large underground facility containing a powerful subspace transmitter, advanced holographic technology, and a system designed to regulate the development of the native Balkosi by introducing aggression-inducing pheromones into their respiratory system. The party further determined that the complex was designed as an elaborate test for visitors. Kirk was confronted with two choices: either poison the Balkosi with a massive dose of pheromones sure to lead to massive violence and, in return, receive advanced holographic systems for the Federation or remove the outside influence on the Balkosi and receive nothing.
After Kirk deactivated the advanced complex and removed the outside influence on the Balkosi's development, a message was transmitted to the Enterprise from an unexplored area of space. It informed the crew that they were suitable subjects for an ongoing test. Kirk and his crew would later discover that the Brassica were behind the events on Balkos. (TOS video game: Judgment Rites "Sentinel")