[TOS] Season 3, Episode 23: All Our Yesterdays
There are a lot of interesting ideas flying around in this episode, but they never quite come together in a way that makes sense.
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Read more at Memory Alpha
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Notes:

Enterprise goes to a planet to warn people about a supernova, but everyone is gone except an old guy and his clones.

Kirk hears someone yell outside, so he runs in that direction, but he goes through a portal. The old guy warns that he hasn’t been properly prepared

Kirk can hear Bones and Spock when he stands next to a wall. People think he’s a witch because the voices talked to him

Kirk finds someone from the future. The people of the planet went to different time periods when they found out their sun was gonna blow.

The hooded person was a lady that was sent there as punishment. She said once someone goes through the portal their body is changed so that they can’t go back.

The same information is said to Kirk, but with some follow up questions, Kirk realizes that he wasn’t changed before going through the portal. I guess Spock couldn’t figure it out.
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