S3E15 · Yesterday's Enterprise

The starship USS Enterprise (NCC-1701-D) encounters a rift in spacetime. The crippled USS Enterprise -C, a ship believed destroyed more than two decades earlier, emerges. The Enterprise -D instantly undergoes a radical change from its previous timeline: it is now a warship, and the United Federation of Planets is at war with the Klingons. Neither Worf nor Counselor Troi are present, and Tasha Yar – killed years before in the original timeline – runs the tactical station. Only Guinan senses that reality has shifted, and meets with Captain Picard. She suggests that the Enterprise -C must return to the past to restore the original timeline. Picard, knowing this would be a suicide mission, refuses to give such an order based on Guinan's intuition alone.

Captain Rachel Garrett of the Enterprise -C and her crew learn they have traveled into the future. Garrett explains that they responded to a distress call from a Klingon outpost on Narendra III, and were attacked by Romulan starships. While his crew repairs the Enterprise -C, Picard and his command staff discuss whether the ship should return to the past. Commander Riker argues that their deaths would be meaningless, but Data suggests that it would be considered an honorable act by the Klingons. Picard discusses the situation with Garrett, revealing to her that the Federation is on the verge of defeat and one more ship will make no difference, but if the Enterprise -C returns to the past they might prevent the war from starting. Garrett agrees and tells her crew that they will return through the anomaly. The two ships are ambushed by a Klingon Bird of Prey; Garrett is killed, and her helmsman, Richard Castillo, takes command.

During the repair efforts, Yar becomes close to Castillo, but is unnerved by tense interactions with Guinan. Guinan reveals that she knows Yar dies a meaningless death in the other timeline. Yar requests a transfer to the Enterprise -C. As the Enterprise -C prepares to return through the anomaly, three Klingon battlecruisers attack. With the anomaly becoming unstable, Picard orders the Enterprise -D to cover the Enterprise -C's withdrawal. The Enterprise -D suffers massive systems damage and major crew losses, including the death of Commander Riker. With the Enterprise -D on the brink of destruction, the Enterprise -C traverses the anomaly, triggering the return of Enterprise -D's previous timeline. Guinan – the only one subtly aware of what has transpired – asks Geordi La Forge to tell her about Yar.