Star Trek: The Original Series, season 1, episode 19
Tomorrow Is Yesterday
Summary
After accidentally traveling back in time to 1969, the Enterprise rescues USAF Captain John Christopher from his crippled fighter jet. The crew struggles to return to their own time, while simultaneously returning Christopher to the Air Force, removing his knowledge of the future and all record of contact with the Enterprise.
Summary from Wikipedia, CC BY-SA 4.0.
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