Star Trek: The Original Series, season 3, episode 8
For the World Is Hollow and I Have Touched the Sky
Summary
As McCoy discovers he is dying of an incurable disease, the crew of the Enterprise rush to stop an asteroid from colliding with a Federation world, only to discover that the asteroid is, in fact, a disguised alien vessel. They find an entire civilization living in the ship who believe they are actually on a planet and a dictatorial "Oracle" who forbids any attempt to discover the truth.
Summary from Wikipedia, CC BY-SA 4.0.
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