Frank Paris — who also wrote episodes for the Western Gunsmoke Gene Roddenberry had compared Star Trek to in his pitch to NBC — wrote “The Aurorals” in the summer of 1968, would have seen Kirk, Spock and McCoy kidnapped by aliens and replaced by androids.
The story was similar to “Wink of an Eye”. In both cases, aliens try to use the crew as breeding stock.
It also bears a resemblance to the Star Trek: The Next Generation episode “Allegiance”, in which Captain Picard is captured by aliens and replaced by an artificial version.
The isolationist planet Auroral is considering joining the Federation. Kirk, Spock and McCoy beam down to assess its bid. They meet Loyung, “rex” of the world. Loyung is a dictator and strangely interested in the personal lives of his guests. Their conversations are monitored by a scientist, Kuyin, who builds android duplicates of the officers. These are send back to the Enterprise.
The android Kirk is quite convincing and orders the Enterprise to continue to its next mission. Loyung informs the real trio that the ship has left and they remain his “guests”. They are each given a beautiful woman for company.
Under heavy guard, the Starfleet officers see no option but to play along. They discover that the girls are terrified of failing in their task, which is to please the men. Loyung still monitors their every move.
The girl assigned to Kirk reveals they are expected to marry the officers the following day. She has a boyfriend who opposes the marriage, but they cannot resist.
The boyfriend turns out to be Kuyin, the scientist, and Kirk manages to elude his guards to speak with him. Kuyin explains that the girls must marry the humans because the people of Auroral need new blood. He also reveals that the three androids aboard the Enterprise have been booby-trapped. They will explode in four hours, destroying the ship.
Before Kirk can discover more, the guards find them. Loyung orders Kirk and Kuyin brought before him. He refuses to stop the destruction of the Enterprise, but finishes the explanation Kuyin started. Hundreds of years ago, the Aurorals achieved racial purity. Now the species is unhappy. He has decided that what they need is variety. Kirk, McCoy and Spock will provide it.
Kirk is disgusted and resists, but a rescue attempt fails.
On board the Enterprise, the crew have started to notice odd behavior from the androids. “McCoy” is strapped to a bed and examined. His android nature — and explosive device — are revealed. The three duplicates are beamed off the ship just in time. They explode in space.
Scotty returns the ship to Auroral. Kuyin knows the Enterprise has survived, but doesn’t reveal this to Loyung. Kirk convinces the scientist to help them construct an android copy of Loyung, one Kirk can control. They use this to break free.
When the Enterprise enters orbit, Loyung is about to ready planetary defenses when his copy emerges. The two Loyungs square off. The android kills the real one.
With the death of Loyung, Kuyin becomes rex of the Aurorals. He promises they will join the Federation and solve their problems without forced interbreeding.