The discovery on the frozen planet Exo III that the missing archaeologist Roger Korby had mastered an extinct civilisation's android technology and planned to spread it secretly through the Federation.
In the Original Series episode What Are Little Girls Made Of?, the Enterprise reaches Exo III to look for Doctor Roger Korby, unheard of for five years. Nurse Christine Chapel, his fiancee, joins Kirk in the landing party. They find Korby living in caverns with his assistant Brown, a woman named Andrea and the towering Ruk, and two security guards are killed.
All three companions are androids. Ruk was built by the planet's vanished Old Ones, and their machinery lets Korby copy a living person. He makes a duplicate Kirk to help him seed androids on a colony world, but Kirk plants a tell-tale phrase in the copy's mind that alerts Spock.
Kirk turns Ruk against his master, and a torn hand reveals that Korby too is now an android, his dying body replaced long ago. Faced with what he has lost, Korby destroys himself and Andrea.