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Alternate Timeline

 * Mar Loc, a Sindareen scientist, uses the Guardian of Forever to travel back to 2368 to kill Counselor Deanna Troi. He succeeds, creating an alternate timeline in which Admiral William T. Riker uses the Guardian to travel back in time to prevent this assassination, while Commodore Data and Lieutenant Blair of the Enterprise-F follow Riker through to prevent the destruction of their own timeline.  Riker succeeds in saving Deanna, thus restoring the future to its previous state.

Stories (in chronological order)
Imzadi: TNG novel