The timestream was changed by sending a Doomsday Machine back in time. There, the Doomsday Machine destroyed several planets, manufactured more Doomsday Machines, and began what is now known as the Doomsday Wars. Captain Kirk, in command of the USS Farragut, is sent back in time to prevent this from happening and save the future. Written by
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Captain James T. Kirk
[adressing an older version of himself
I'm... guessing that you're from the future... Admiral?
[Ambasador Spock enters frame
That is a logical assumption. 2373 to be precise.