An earlier Enterprise-C emerges from a rift and changes history.
The Enterprise-D encounters a rift in space, and out of it comes the Enterprise-C, a starship thought to have been lost decades earlier. Its arrival changes history, and the Enterprise-D becomes a warship in a Federation that has been at war with the Klingons for years. Guinan senses that something is deeply wrong with the new timeline.
Picard must decide whether to send the Enterprise-C back to its certain destruction in order to restore the original timeline. The story, one of the most celebrated of the series, includes the return of Tasha Yar, who is alive in this version of events. It is a powerful story about sacrifice and the cost of peace, and it leads to the Romulan commander Sela in a later episode.