1-26: Errand of Mercy
Kirk and Spock try to protect a seemingly primitive species from a Klingon invasion.
On a planet resembling 1960s Earth, all of the adults have perished from a deadly disease.
1-05: The Enemy Within
A transporter malfunction creates two distinctly opposite Kirks.
1-25: The Devil in the Dark
The Enterprise encounters a silicone-based life-form who is attacking pergium miners.