Star Trek The Next Generation: Yesterday's Enterprise

In Ten-Forward, Guinan introduces Worf to prune juice, a drink which he enjoys immensely (calling it "A warrior's drink"). He is suddenly called to the bridge.

The Enterprise-D encounters a rift in space-time that appears to have no discernible event horizon; it both is, and is not, there. After a brief moment a battered and damaged ship comes out of the rift, transmitting a distress call and identifying itself as the USS Enterprise-C, a ship lost with all hands twenty-two years in the past and the predecessor to the Enterprise-D. After the Enterprise-C emerges from the rift, a dramatic shift occurs on the bridge of the Enterprise-D: the bright, open-spaced bridge is replaced by a dark military bridge with multiple tactical stations, Worf and Counselor Troi are missing, and Lieutenant Tasha Yar, killed in the episode "Skin of Evil", is chief tactical officer. The Federation is at war with the Klingon Empire and the Federation is losing; indeed, the Federation is barely six months away from surrender.

Guinan is the only one who appears to notice the change, not knowing for sure what is happening, but knowing that "something is wrong". She approaches Captain Picard about her thoughts and, while not entirely dismissive of her feelings, he cannot act on them without more evidence.

Captain Rachel Garrett (Tricia O’Neil), commanding officer of the USS Enterprise-C, wakes up in sickbay and is startled at the modern Starfleet uniforms and the advanced medical bay the likes of which she has never seen before "even on a Starbase", even though Captain Picard initially relayed orders that no one from the Enterprise-C is to know about "where, or when they are", Captain Garrett’s perceptions of her surroundings force Captain Picard to acknowledge the inevitability of them discovering the truth. When asked how they got here, Garrett tells Picard of how they had responded to a distress call from the Klingon outpost on Narendra III. On arrival they found that the outpost was being attacked by 4 Romulan Warbirds which they engaged. The last thing she remembers is a fierce volley of photon torpedoes and a burst of bright light.

Captain Garrett initially wants to remain in the future and assist the Federation with the Klingon War, but Captain Picard informs her that the war is going badly for the Federation, "far worse than is generally known." Starfleet is on the brink of collapse and one more ship will not help in the present. Captain Picard informs her that if what Guinan believes is true, that there was not supposed to be a war, then the Enterprise-C may have had a role in the peace that should be in place and is in place, in the alternate timeline.

It is decided, against the objections of Commander Riker, that the Enterprise-C should return to its own time and perhaps, by doing so, alter the course of history as it has unfolded in this alternate timeline. While Captain Picard and Captain Garrett are aboard the Enterprise-C supervising the final repairs, a Klingon scout ship locates the two ships, opens fire, and quickly makes her escape to inform the Klingon High Command of the location of the two Federation Starships.

The Enterprise-C is caught with shields down and suffers moderate damage. Captain Picard returns safely to the Enterprise-D, but Captain Garrett is killed in an explosion on the Enterprise-C's bridge.

Lieutenant Yar notices the strange way that Guinan, the enigmatic and wise bartender, reacts to her presence for the first time seeing her since she noticed the ship's change, and, against her objections, tells why she reacted so strangely. Guinan tells her that she never actually met Tasha Yar before coming aboard the Enterprise-D; all she knew about her was that she died a meaningless death. Because of this revelation, Yar asks Captain Picard for permission to transfer to the Enterprise-C as a tactical officer. Captain Picard emotionally acquiesces to her request when Yar objects to dying a meaningless death.

Lieutenant Richard Castillo (Christopher McDonald), the last remaining Enterprise-C bridge officer and a budding romantic interest with Tasha Yar, assumes command of the Enterprise-C and prepares to take the ship back through the rift—which is now growing unstable—to reenter combat with the Romulans attacking the Klingon outpost.

As the Enterprise-C is entering the rift three Klingon K'vort-class battlecruisers attack the Enterprise-D and outflank it. There is a battle in which the Enterprise-D manages to destroy one of the battleships, while protecting the Enterprise-C. However, in order to protect the Enterprice-C, the Enterprise-D is forced to stay within 200 kilometers of its predecessor, rendering it unable to maneuver properly during combat. As a result, the ship suffers major damage and is close to destruction. The bridge erupts in fire and Commander Riker suffers what appears to be a fatal wound when a console explosion embeds metal fragments into his neck. The Klingons demand the Enterprise's surrender. Picard sneers at this request and mutters defiantly to himself, "That will be the day”. He leaps up and takes over Riker’s station and fires the last few rounds of phaser fire as he tries to buy the Enterprise-C more time. The Enterprise-C manages to enter the rift successfully and in another striking change, the Enterprise-D is returned to normal and the rift that altered time vanishes before the Enterprise-D can scan it. Worf and Troi are back and the Enterprise-D is on its way to meet a Klingon delegation. In the last scene, Guinan asks the bridge if all is well, and then says to Geordi La Forge: "Tell me about Tasha Yar."