Exo III was once a lush planet until the system's star began to dim and temperatures on the planet got lower with each passing year. This was due to star's energy being drained by the One during his battle with Q. (TNG novel: Q-Strike) The population was forced to go underground, where they created androids to serve them. However, the androids developed self-awareness and killed their masters. Over the next 500,000 years the androids themselves began to wither away, and many put themselves in stasis until needed again, with Ruk tending to them.
In 2261, the Federation archaeologist, Dr. Roger Korby, and his assistant Dr. Aaron Brown, crashlanded on the planet. While Dr. Brown died, Ruk was able to rescue Dr. Korby, although he was badly injured. Korby became interested in the androids' capability to build androids and had an android duplicate of himself and Dr. Brown created, as well as a female associate named Andrea.
In 2266, the USS Enterprise arrived at Exo III in an attempt to ascertain the location of Korby's expedition. After creating an android duplicate of Captain James T. Kirk in an attempt to spread the androids across the Federation, Korby's master plan was stopped. (TOS episode: "What Are Little Girls Made Of?") Another attempt by the Brown android in 2268 led to similar disaster, and Exo III was abandoned. (TOS novel: Double, Double)
In 2304, Dr. Emil Vaslovik, Noonien Soong, and Ira Graves traveled to Exo III and rediscovered Dr. Korby's home on the planet. During their investigations, they accidentally released several androids left in stasis, and they spend several decades examining the wreckage of Dr. Korby's ship and begin to build their own starships, allowing them to leave Exo III. (TNG novel: Immortal Coil)