Tomorrow and Tomorrow and Tomorrow
Summary
A mysterious wounded man appears on the Enterprise, gives a time travel device to La'An, and dies. La'An finds herself in an alternate timeline where Earth is ravaged by war and the Enterprise is captained by James T. Kirk. The device transports her and Kirk to 2020s Toronto, where the pair form a romantic bond. They search for the point of divergence that caused the alternate timeline with help from a younger Pelia, and find a eugenics lab where a young Khan Noonien Singh is being raised. A Romulan time traveler named Sera intends to change human history by killing Khan and preventing the Eugenics Wars, which would keep humanity from the path that led to unity and the Federation. Sera kills Kirk and fights La'An, who kills Sera and saves Khan.
Summary from Wikipedia, CC BY-SA 4.0.
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