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?").
Months after the first Exo III incident Harry Mudd had learned about Exo III and the android manufacturing plant on it, and managed to join one of the Federation research teams that had been assigned to Exo III to study the technology of the Old Ones.
Using the original Andrea android template Harry created several hundreds more which he sold as entertainers to various worlds throughout the galaxy including Wrigley's Pleasure Planet, as well as a number of Ruk androids which he used to capture the other Federation researchers.
On Stardate 4991.3 Kirk and the Enterprise crew became aware of Harry's latest scheme and returned to Exo III to put an end to it. It let to a confrontation with the Ruk androids who like the original sought to preserve themselves by killing Captain Kirk, Commander Spock, Doctor McCoy, Chief Engineer Scott, and Harry Mudd before they were destroyed by security personnel.
Soon after Commander Spock and Lt-Com Scott shut down the android manufacturing plant permanently when Harry Mudd sought to create new Ruk models he could control. (TOS comic: "The Survival Equation")
In 2268 another model of the Brown android who had not been present during first confrontation between the Enterprise crew and Roger Korby made another attempt at carrying out Dr Korby's plan. After this plan was stopped as well 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)
There is a discrepancy between the events of New Visions 8 and those of Double Double and Immortal Coil as the android manufacturing plant and the connection between the androids that kept them active and functional was shut down permanently. In the later novels which were published before New Visions 8 this connection was never mentioned and probably did not exist.