Upon returning from a mission to scan the Guardian of Forever, Spock discovers that there is no record of him serving in Starfleet and he has been replaced by an Andorian first officer, Thelin. Neither Spock nor anyone else who were on the planet have a memory of the Andorian, while no one on the Enterprise remembers Spock! Computer records indicate Spock died as a youngster during a Vulcan ritual, and that they were scanning the Guardian when it was ""running"" that period of time. Since Spock didn't go through to save himself, he altered the timeline. The Guardian sends Spock through where he pretends to be a distant cousin. He meets Amanda and Sarek and an embittered young Spock. Young Spock eventually tries to undertake the desert ritual on his own and is almost killed. His pet sehlat saves him at the cost of its own life before Old Spock can show up. The timeline restored, everything is put back to normal.
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