The Changeling

TOS Season 2 Episode 3
Air Date: September 29th, 1967

The responds to a distress call and when they arrive in the sector, four billion people over four planets are gone. The Enterprise is then attacked with a weapon they can't avoid and after only a handful of hits risks destruction. The Enterprise tries to communicate with whomever is attacking and the response is digital. Eventually communication works and they discover it is from an old Earth probe called Nomad that was damaged and repaired by some other beings.

Nomad's memory banks were corrupted and it mistook Kirk for his creator. His mission is finding perfect life and it appears destroying imperfect life. Kirk is able to keep Nomad under control for a while but it does wipe Uhura's mind and kills Scotty for a short time (almost 2 red shirt deaths, if not for the fact they are main characters). Kirk allows Spock to try to mind meld with the machine, which indicates it is sentient and not just mechanical parts. It ends up taking over Spock's body for a moment during the meld.

After this Nomad decides to go exploring and kills some security officers that tried to shoot it. It heads to engineering and starts fixing parts of the Enterprises engines considering them imperfections. He gets the warp engines to go faster then warp ten. On his way back to the brig he kills two more security guards.

Kirk tricks Nomad into realizing that making mistakes is part of imperfection and then shows Nomad that it made a mistake in who it's creator is therefore it must destroy itself.

This episode saw a record four red shirts die.