Star Trek: The Original Series, season 1, episode 13
The Conscience of the King
Summary
While visiting an old friend, Kirk suspects a Shakespearean actor may actually be the murderous former governor of Tarsus IV ("Kodos the Executioner"), where Kirk grew up. Kirk invites the acting troupe aboard the Enterprise to investigate. Spock discovers that there had been only nine people who could have identified Kodos, but seven of them were murdered when the troupe was nearby. He is certain that Captain Kirk and Lieutenant Riley, the only remaining witnesses, are the next targets.
Summary from Wikipedia, CC BY-SA 4.0.
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