[[caption-width-right:350:Tasha! You're ''alive!!!'']]
Guinan is introducing Worf to the wonders of prune juice when the ''Enterprise'' comes upon some sort of space-time disturbance. Suddenly, a ship emerges--the USS ''Enterprise'' (NCC-1701-'''C'''), displaced 22 years in time. And it has survivors.

Panning back to the bridge of the ''Enterprise''-D, things are... different. The lighting is darker, the design a bit more sinister, the uniforms are a slightly different design, everyone is armed, and Wesley is a full member of Starfleet--and holy crap, is that Tasha Yar standing there where Worf ought to be? And where's Deanna? Worf's absence is easily explained: the Federation has been at war with the Klingons for over 20 years, a war which they are now on the brink of losing. They desperately need any ship they can get, and the ''Enterprise''-C, though badly damaged, looks repairable.

They bring the survivors on board, including the moderately injured Captain Garrett. She tells Picard and Dr. Crusher that they had been responding to a distress call from a Klingon outpost at Narendra III when they were set upon by four Romulan Warbirds and were moments from destruction when they fell into the rift. It's a pity, really--just think what beneficial effects a Federation rescue of a Klingon base might have had on the negotiations then taking place. Why, 20-some years of devastating war might well have been averted!

Alone of the crew of the ''Enterprise'', Guinan's BizarreAlienSenses leads her to pick up on the shift in the timeline, though she cannot fully comprehended it; she just knows that ''something'' is ''off''. She talks to Picard about it, but isn't able to give him anything more definite than that this isn't what's supposed to be happening, and implores him to send the ''Enterprise''-C back through the rift to where it belongs. Picard is understandably reluctant to send 125 people into certain death based only on a few cryptic remarks from his bartender, but Guinan points out that the war they are in now has by this point claimed the lives of a little over 40 billion people; a war which should not have been happening in the first place. The only way to save those billions is to sacrifice that small group of people. Picard still feels that such an act would go against everything he stands for, but Guinan begs him to trust her.

Meanwhile, Tasha has been liaising with the acting second in command of the ''Enterprise''-C, Lieutenant Junior Grade Richard Castillo, trying to get the older vessel combat ready once more. The two of them clearly like each other, even though all the very strange looks Guinan keeps giving her are a little disconcerting. Meanwhile, Picard informs the other officers that he has decided to send ''Enterprise''-C back to the past where it belongs. Dr. Crusher is incredulous, when Picard discloses that he has taken this decision based only on Guinan's intuition. Geordi wonders how Guinan could possibly know the timeline has charged altered if she has been altered along with everyone else. Data suspects that her species has a perception which goes beyond linear time. Riker, meanwhile, questions the logic of such the act, arguing that the ''Enterprise''-C has no possible way of saving Narendra III and the only thing it would really accomplish is sending the crew to their deaths, but Data points out that the Klingons have an awful lot of respect for people who die in battle, and dying in the defense of the Klingon outpost would likely be seen as a very honorable act by them.

Picard goes to the ''Enterprise''-C to discuss the situation with Garrett, who tells him that she and her crew, even if they have been displaced in time, are still with the Federation and are eager to serve it again. Picard then informs her of his decision and how he has reached it. Garrett asks if he really trusts Guinan's judgement that much, and Picard answers that he learned long ago to trust his old friend's wisdom. Garrett, of course, knows just as well that there is no chance for survival if they go back alone to the past, and suggests the ''Enterprise''-D could return with them, and Picard says they cannot. Garrett accepts that, saying Picard probably as belong little in her time as the ''Enterprise''-C's crew belongs in Picard's. But Garrett has told her crew that the Federation needs another ship against the Klingons and they need to get used to the idea. In response, Picard quietly reveals a bit of classified information to her: the war is looking much, ''much'' worse than what is generally known. The Federation stands on the verge of defeat in the war and Starfleet Command believes that it will happen within six months. Just one starship is not going make any impact in the present, but 22 years ago, one ship could stop the war before it starts.

Realizing that Picard is right, Garrett tells Castillo to inform the crew they will go back through the anomaly to their own time. She promises Picard she and the crew will make sure the Romulans get a good fight, and that history will remember their actions. Picard tells Garrett he knows they will. As Picard beams back to the ''Enterprise''-D, Yar and Castillo say their goodbyes, but they are interrupted as the two ships are suddenly attacked by a passing Klingon Bird-of-Prey on a scouting mission. The ''Enterprise''-C sustains some damage in the fight, resulting in Garrett getting killed by a piece of shrapnel, leaving Castillo to take command. Castillo makes preparations to carry out the mission in her stead, and he and Tasha share a tender goodbye. Tasha confronts Guinan about the aforementioned strange looks, and Guinan admits that in the "correct" timeline, [[Recap/StarTrekTheNextGenerationS1E22SkinOfEvil she died horribly and pointlessly]]. Upon hearing this cheerful news, Tasha decides to transfer to the ''Enterprise''-C and take her chances with the Romulans, where at least her death might make a difference. This couldn't possibly have any consequences down the road at all.

Those pesky Klingons still won't go away, though, and as the ''Enterprise''-C limps back towards the rift, the ''Enterprise''-D devotes itself wholly to defending its doomed predecessor. Harried by three battle cruisers, the ''Enterprise''-D suffers major crew losses, including Riker, forcing Picard to man the tactical station himself. The Klingons hails the ''Enterprise''-D, demanding their surrender to them. "That'll be the day," scoffs Picard as he continues to fire off all working phasers against them, even as the bridge becomes engulfed in flames. The Klingon ships batter away at the now-helpless ''Enterprise''-D, and with a warp core breach imminent, it looks like the end for both ships. Just as the ''Enterprise''-D is about to be destroyed, the ''Enterprise''-C makes it into the rift--

...and everything returns to normal. Picard stands on the bridge in the exact spot he was before the change in timeline. He asks for a report. Worf, back once again in his usual spot, reports that his sensor readings fluctuated and what appeared to be a ship has now vanished. Data reports the anomaly is closing in on itself. Just as Picard orders Wesley to plot a new course, Guinan calls up over the intercom and asks if everything is all right. Surprised by her call, Picard answers somewhat bemused that everything is fine on the bridge and he asks her if there is something wrong. Guinan, glancing at a well-lit, happy Ten-Forward, says that nothing is wrong and apologizes if she bothered them. She then sits down [[CrowningMomentOfHeartwarming to ask Geordi about Tasha Yar]].
!!Tropes in this episode include:
* AbsenteeActor: Marina Sirtis (Counselor Troi) only appears in the two short bridge scenes on the "regular" ''Enterprise''--one before and one after the main plot of the episode--and has no dialogue. Understandably, Worf also appears only in the normal timeline scenes.
* AnyoneCanDie: The best thing about an AlternateTimeline episode. Captain Garrett and Riker are both killed by ExplodingInstrumentation, and planned deaths of more of the bridge crew were cut for time.
* BadPresent: Or BadFuture--it is a BadFuture story in everything but the fact that bad alternate time is the present from the perspective of the main characters.
* BadassBoast: "Let's make sure history never forgets... the name... '''''Enterprise'''''."
* CallBack: Worf's spiel about human females being too fragile for [[InsistentTerminology "companionship"]] is a call back to a nearly identical conversation he had back in season one's "[[Recap/StarTrekTheNextGenerationS1E7Justice Justice]]", there with Commander Riker, but here with Guinan. It would be playfully revisited again later on in ''Series/StarTrekDeepSpaceNine''.
* CaptainsLog: Picard gives his usual voiceover, but in the alternate timeline he's recording a "military log" and using "combat dates" rather than Stardates.
* ChildSoldier: Wesley Crusher in the alternate timeline is a full ensign and a member of the ''Enterprise'' bridge crew, showing how desperate the Federation must be getting in their war with the Klingons.
* CoolStarship: The ''Ambassador''-class ''Enterprise''-C. Notable in that the ''Ambassador'' only made three other appearances in the franchise and it remains an extremely popular ship in the fandom.
* CriticalStaffingShortage: Riker points out that if the ''Enterprise''-C goes back, Lt. Castillo will have "limited support from Ops, no Tactical support, reduced staff in Engineering..." before Castillo cuts him off.
* DefiantToTheEnd: Picard, given the offer to surrender to the Klingons, spits out a FacingTheBulletsOneLiner, vaults over the Tactical rail to man Riker's empty station, and continues attempting to fire the phasers as the bridge catches fire around him.
* DrinkOrder: Worf's love of prune juice begins here.
* ExpendableAlternateUniverse: Discussed.
-->'''Guinan:''' I can't explain it to myself so I can't explain it to you. I only know that I'm right.\\
'''Picard:''' Who is to say that this history is any less proper than the other?\\
'''Guinan:''' I suppose I am.\\
'''Picard:''' Not good enough, damn it! Not good enough!
* FacingTheBulletsOneLiner: The ''Enterprise''-D is crippled, with no chance of winning the battle. The Klingons offer to let the ''Enterprise'' surrender.
-->'''Captain Picard:''' That'll be the day.
* FixFic: Denise Crosby left the series unexpectedly while the first season was being filmed, which meant Tasha Yar had to be written out. Many viewers criticized Yar's demise in "[[Recap/StarTrekTheNextGenerationS1E22SkinOfEvil Skin of Evil]]" as being abrupt and pointless. When Crosby returned for this episode, the writers got a chance to give her a more heroic death. This is [[LampshadeHanging lampshaded]] in the dialogue between Yar and Picard, when he says that instead of having a "senseless death" she wants her sacrifice to "count for something."
* {{Foreshadowing}}: The Bad Future ''Enterprise''-D is destroyed [[InSpiteOfANail in the exact same way]] as the ''Enterprise''-D we've been watching through the show will be: Klingon Birds of Prey damage the warp engine, which causes a warp core breach. In fact, it's so precisely identical, the same panel explodes and the core starts leaking from the same spot. Geordi's dialogue in ''[[Film/StarTrekGenerations Generations]]'' is lifted almost word for word from this episode, with just the time until explosion changed between the two versions. Of note, both this episode and ''Generations'' were directed by David Carson.
* ForWantOfANail: The whole premise of the episode; the ''Enterprise''-C turns out to be a very important nail indeed. Without a HeroicSacrifice on the part of a Federation ship in defense of a Klingon outpost, negotiations between the two sides break down into a 20-year war.
* GettingCrapPastTheRadar: And not even trying very hard. Guinan tries to talk Worf into seeking... [[YouNeedToGetLaid companionship]] from other members of the crew. Worf states that human women are [[DestructoNookie far too fragile]], and that he would require a Klingon woman for... [[InsistentTerminology companionship]].
* HeroicSacrifice: The original fate of the ''Enterprise''-C, fighting four Romulan warbirds in defense of a Klingon colony before the Klingons and Federation were allies. Giving their lives in a doomed attempt to answer the colony's distress call was (as Data pointed out) an act of courage and honor so impressive that it made the Klingon Empire reconsider their decades of hostility towards the Federation. Also the fate of the {{Bad Future}}'s ''Enterprise''-D.
* HoldTheLine: The ''Enterprise''-D has to hold off three Klingon Birds of Prey long enough for the ''Enterprise''-C to re-enter the temporal rift and reset history. Similarly, the ''Enterprise''-C has to last long enough in the Battle of Narendra III to prove to the Klingons that the Federation are TrueCompanions.
* InSpiteOfANail: Played with. Starfleet is in a hopeless war against the Klingon Empire, without any civilians aboard the ''Enterprise'' except Guinan, but most of the crew we know is there, though Wesley in this reality is an actual ensign instead of an "acting ensign" as he was at the time.
* MildlyMilitary:
** The fact that this trope is {{Averted| Trope}} is one of the earliest indicators to the audience that something is off. All of the crew carry phasers aboard the ship, the lighting is dimmed for a darker mood, the ramp leading to the back of the bridge has been replaced with steps, there are fewer chairs on the Bridge for officers to lounge in ([[NumberTwo Riker]] now stands behind the railing instead of sitting at the Captain's side), the replicators produce standardized military rations, the ship doesn't have a counselor on the bridge--Troi doesn't appear in the Bad Future at all--and the ''Enterprise'' is even [[InsistentTerminology repeatedly referred to]] as a "battleship" instead of a "starship". The ''Enterprise''-C, meanwhile, is referred to as a cruiser. Even the uniforms are slightly different, now having a [[HighCollarOfDoom closed collar to make the final frontier just that ever bit less friendly]].
** There's also the frosty interaction between Picard and Riker. Riker is not referred to as "Number One" in this timeline, and on more than one occasion bluntly challenges Picard's decisions in front of the crew. Picard is noticeably more emotionally removed from the other officers, giving the atmosphere on the ship more of a tense feeling.
* MundaneMadeAwesome: Prune Juice... ''A Warrior's Drink!''
* MySignificanceSenseIsTingling: Guinan senses that something's wrong when the NegativeSpaceWedgie first appears, and it gets worse when the timeline changes. It also flares up whenever she's near Tasha, since she's supposed to be dead.
* TheNeedsOfTheMany: The ''Enterprise''-C must return to prevent a war which kills ''billions''.
* NegativeSpaceWedgie: Apparently caused or exacerbated by all the weapons discharges during the Battle of Narendra III.
* OhCrap: La Forge delivers a well-warranted one. Doubly warranted if you consider that the ''Enterprise'''s plasma coolant is [[BodyHorror horrifically corrosive]] to organic matter.
-->'''La Forge:''' Coolant leak! Bridge, we've got a coolant leak in the engine core! I can't shut it down; I estimate two minutes until a ''warp core breach!''
* RememberTheNewGuy: Lampshaded--all the other crew remember Tasha as having been with them all along, but Guinan, who joined the ship after Tasha died, can't remember her previously existing.
* ResetButtonSuicideMission: The ''Enterprise''-D sacrifices itself to cover the return of the ''Enterprise''-C back to its original time in order to prevent the alternate timeline it emerged into from occurring.
* RippleEffectIndicator: Worf arrives on the bridge, and announces something strange on sensors. Time Ripple ensues, and we cut to Tasha Yar (having died previously on the show) on the bridge in Worf's place. Once the timeline is restored, Worf is back where he should be.
* RippleEffectProofMemory: Guinan only, and [[DownplayedTrope downplayed]] at that: She doesn't seem to know '''exactly''' what's wrong with the timeline ("I know it's the same bridge; I also know it's ''wrong''!"), but she has a good idea of how to fix it.
* SacrificialLion: One for each space battle shown: Captain Garrett and Commander Riker.
* SetRightWhatOnceWentWrong: The ''Enterprise''-C's mission. The crew of the ''Enterprise''-D set forth to help them and protect them until they can go back.
* StatusQuoIsGod: Obviously done so we could have a cast we're familiar with, but given that the premise involves the prior 20 years of history to be drastically different, with the Federation entangled in a decades-long war, it's highly improbable that any of the original crew would have ended up on the ''Enterprise''-D, and especially unlikely that more than one or, at most, two, would have. Instead, with the exception of Worf's swap with Yar (due to the Klingon war), and Troi, everyone else is not only stationed on the same exact ship, but also in their exact same positions.
* TrademarkFavoriteFood: As noted above, in the teaser to this episode Guinan introduces Worf to the wonders of prune juice, which he immediately dubs "a Warrior's Drink." Prune juice will remain his drink of choice all the way through his time on ''Series/StarTrekDeepSpaceNine''.
* WhatHappenedToTheMouse:
** Whatever became of Tasha Yar after going back into the past? This question gets answered [[Recap/StarTrekTheNextGenerationS4E26S5E1Redemption later on in the show]].
** Meanwhile, no mention is made in the BadPresent of Mr. Worf, who quietly disappears between scene changes once the NegativeSpaceWedgie appears.
** The novel ''Q-Squared'' provides part of answer to this: Worf is rescued by his own people and grows up "standard" Klingon. The treason charge isn't slapped on himself and Kurn since, with Mogh as an active House, they don't make quite as convenient scapegoats. Worf becomes captain of a warship and a celebrated destroyer of Federation ships. The novel also explains Troi's disappearance from the episode as well: the Betazoids had been wiped out by the Klingons.
** The shooting script had indicated that it could have been Worf on the other end of the commline, instead of a random Klingon. One wonders why they changed their mind.
* WhatYouAreInTheDark: Inverted. If the ''Enterprise''-D is successful in helping the ''Enterprise''-C return to their time, the crew of the ''Enterprise''-D will have no idea what they did and what it meant for the Federation. They still do it, because it's the right thing to do. Although, back in the in fixed timeline, Guinan knows and later tells Picard.
* WithAllDueRespect: Riker to Picard in regards to sending the ''Enterprise''-C back through time to correct the past that altered the present.
-->'''Riker:''' With all due respect, sir, you'd be asking 125 people to die a meaningless death.
* YouAreInCommandNow: Lt. Castillo, the only remaining bridge officer, takes command of the ''Enterprise''-C after Captain Garrett is killed.
* YouShallNotPass:
** The ''Enterprise''-D's HeroicSacrifice to allow the ''Enterprise''-C to get back to her own time.
--->'''Captain Picard:''' Attention all hands. {{As you know}}, we could outrun the Klingon vessels. But we must protect the ''Enterprise''-C until she enters the temporal rift. [[LetsGetDangerous And we must succeed.]] Let's make sure history never forgets... the name... ''Enterprise''. Picard out.
** And of course, once the timeline is restored, the final fate of the ''Enterprise''-C, destroyed after taking on four Romulan warbirds in defense of a Klingon colony. Never forget the name ''Enterprise'' indeed.
->'''Guinan:''' Geordi, tell me about... Tasha Yar.