TNG Season 3 Episode 15: Yesterday’s Enterprise

You know something major has occurred when the end of the pre-credit sequence involves Worf being replaced by Lt. Yar… The USS Enterprise NCC-1701-C, the Ambassador Class predecessor to the Galaxy Class Enterprise-D, turns up after going missing and being presumed destroyed for 22 years. Unfortunately Guinan’s extra-normal perception detects that the timeline has changed.
This story sets up a few things, as well as being downright brilliant in its own right, and is every bit as much a must-watch as Best Of Both Worlds.

Plot

The starship USS Enterprise (NCC-1701-D) encounters a rift in spacetime. The crippled USS Enterprise-C, a ship believed to have been destroyed more than two decades earlier, emerges. The Enterprise-D 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 – is running the tactical station. Only Guinan senses that reality has shifted, and meets with Captain Picard. She suggests that the Enterprise-C does not belong in their time and must return to the past. 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 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 and tends to the crew's injuries, 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, who tells him that her crew will serve the Federation in the present. Picard reveals to her that the Federation is on the verge of defeat and the presence of 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, but 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, which is granted by Picard. 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.[1]