Star Trek: The Next Generation/Recap/S3/E15 Yesterday's Enterprise

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 Enterprise 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 and Wesley is a full member of starfleet - and holy crap, is that Tasha Yar standing there where Worf ought to be? 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 reparable.

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 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 devestating war might well have been averted!

Alone of the crew of the Enterprise, Guinan has noticed that something is Wrong. 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 124 people into certain death based only on a few cryptic remarks from his bartender, but Guinan begs him to trust her. And then the Klingons turn up.

Meanwhile, Tasha has been liaising with the second in command of the Enterprise C, trying to get the older vessel combat ready once more. The two of them clearly like each other, and she is only a little offput by all the very strange looks Guinan keeps giving her. Picard finally decides to send the Enterprise C back where it belongs, on the logic that even if it is a suicide mission, Klingons have an awful lot of respect for people who die battle, and they're all just going to get killed anyway when the present day Klingons turn up. Which they promptly do.

In the ensuing battle, both ships are badly damaged and Captain Garrett is killed by a piece of shrapnel. Tasha confronts Guinan about the aforementioned strange looks, and Guinan admits that in the "correct" timeline, 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 Birds of Prey, and with a 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. The episode ends with Guinan glancing about a well-lit, happy Ten-Forward, and she sits down to ask Geordi about Tasha Yar...

Tropes:

 * Bad Future
 * Growing the Beard: one of the key episodes that made Season Three so memorable.
 * Hey, It's That Guy!: The surviving bridge officer, Lt. Castillo, is played by Christopher McDonald!
 * Negative Space Wedgie
 * Ripple-Effect-Proof Memory: Guinan only.
 * You Already Changed the Past