Star Trek: The Original Series, season 1, episode 9
Dagger of the Mind
Summary
While the Enterprise is on a resupply mission to a rehabilitation colony for the criminally insane, a former doctor — now insane — sneaks on board the ship. Beaming down to the planet, Kirk and a female crew member discover that the chief doctor has been using a device that destroys the human mind. Spock performs a mind meld for the first time in this episode.
Summary from Wikipedia, CC BY-SA 4.0.
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