Twilight Sparkle fights her future, and Spike doesn't like the looks of his own.
Season 2, Episode 20: "It's About Time"
Spike is awakened late one night (out of what we can probably assume is just one of many dreams about Rarity) by the sound of Twilight Sparkle doing what she does best: Stressing out over little things. In this case, the little thing is finding time in her schedule for more scheduling. Hey, it's what she does.
She pulls an all-nighter and solves the dilemma, but her victory is rudely interrupted by what appears to be space-time anomaly. Hey, it's what they do.
From it, emerges another Twilight Sparkle ...
... a Twilight Sparkle who is injured, eye-patched, bandaged, black-bodysuited, shorn of mane, and not scientifically possible.
Scientifically Impossible Twilight Sparkle claims to have a message from the future, a future which Scientifically Plausible Twilight Sparkle surmises must be a far-flung time of an epic pony war, given the former's ragged appearance. (Also, hey, ponies have epic wars!)
Nope. She's from next Tuesday, but that's not important right now.
By the way, kids, if you don't know the origin of "But that's not important right now" as a comic construct, please finish reading this recap, patronize one SF Weekly's fine sponsors, and then watch a movie called Airplane!, which will be the best $2.99 you ever spent to watch a movie on YouTube.
Ragged Twilight is trying to give Well-Groomed Twilight important news, but the latter keeps bombarding the former with questions about time travel. Turns out the relevant spells are in the Star Swirl the Bearded wing (continuity!) in the Canterlot Archives, and now if Today's Twilight would just shut up, Next Tuesday's Twilight can tell her to be sure not to --
Brrrzap! Preview Twilight disappears, and Twilight du Jour is left alone to try to figure out what it all meant. And since it's what she does even when she isn't horribly sleep-deprived, she assumes the worst: Tuesday next, disaster will strike. (And so Twilight's adversarial relationship with Tuesdays continues.)
Twilight rallies the rest of Ponyville (an easily rallied town, as River City's descendants always are) to prepare not only Ponyville but all of Equestria from Fillydelphia to Las Pegasus for the unspecified disaster. Twilight remains unconvinced, though.
And reasonably enough, since given the sudden appearance of the ginormous three-headed dog Cerberus, whom Twilight recognizes as being the guardian of the underworld dungeon of Tartarus. Since Cerberus is no longer on the job, the imprisoned evil creatures will escape and destroy Equestria. So that's the disaster!
The day is saved by Fluttershy's way with animals ...
... and using one of the balls Pinkie Pie keeps stashed all over Ponyville in case of ball emergency, Twilight escorts Cerberus back to the gates of the underworld. Just let that concept rattle around in your brain for a while.
She returns from her unseen/epic adventure, a little scuffed but otherwise feeling triumphant ... and then she gets a paper cut in a familiar place. The disaster is still coming!
Twilight decides to do nothing at all, but that doesn't work either, since Spike's breath inadvertently burns off half her mane. (The fact that he's also begun an ice cream binge to take advantage of Twilight's distracted state doesn't make his breath any less fiery.)
Since her preventative measures aren't preventing the future like she'd hoped, Twilight, now many days without sleep, is monitoring everything -- as in, everything in life and the universe. But not only is it Monday night, meaning the disaster will strike tomorrow, Twilight absent-mindedly looks at the sun through her telescope. Enter the eyepatch (one of the many Pinkie has stashed all over Ponyville in case of eyepatch emergency)!
There's only one thing left to do: Put on black bodysuits, break into the Star Swirl the Bearded Wing at the Canterlot Archives, and find a spell to stop time. Obviously.
It's a stealth mission which requires no stealth whatsoever, since it's not illegal to walk through Canterlot at night, and the guards are happy to let their buddy Twilight Sparkle in.
The asked question: Where is the time-stopping spell? The unasked question: Is there a time-stopping spell?
They can't find it, Tuesday arrives, and ... nothing happens. It's a perfectly lovely morning, and Princess Celestia even makes a cameo, just because.
Celestia leaves before Twilight can deliver the Shoehorn: There's no point in worrying about the future.
They never did find that time-stopping spell, but Pinkie found a more generic single-serving time-travel spell, so, Present Twilight decides to go back and tell Past Twilight not to worry so much ...
... but, as we saw at the beginning of the episode, Past Twilight won't let Present Twilight get a word in edgewise. As Pinkie wisely points out, that's not Present Twilight's problem anymore.
Still, it's hard to not feel a little existential terror when conisdering that time loop continuing on: Past Twilight will freak out, realize she was overacting, and eventually go back in time to try to warn her Past Twilight not to worry, and that Past Twilight will freak out, and ...