I'm going to warn you, I really didn't care for this episode. Watching it through the first time was fun, but once it was done, the whole just didn't add up to the sum of its parts for me.
Twilight Sparkle and Spike are working on some advanced magic, but keep getting distracted by Pinkie Pie, who is darting around like a maniac with an umbrella hat on her head. It's due to her tail being twitchy, which, as she informs Twilight and Spike, means something is about to fall from the sky. She warns them to take cover, but Twilight scoffs; there's barely a cloud in the sky, so it can't be about to rain. But Pinkie wasn't referring to rain, as she finds out when a frog lands right on her snout. Strange weather conditions? A sudden plague? Actually, it was Fluttershy's doing, as she's transporting a bunch of frogs from the pond near her house to a bog on the other side of town. She flies off sans one frog, and Twilight heads off to wherever Pinkie isn't to finish practicing.
Spike is agog over Pinkie's prediction skills, but Twilight chalks it up to coincidence, and scoffs at Pinkie's warning that her tail is still twitching. It is, in fact, mid-scoff that she falls into a ditch, ending Pinkie's twitchy tail. Despite this, she's still not convinced of Pinkie Pie's powers, and tells this to Spike and Applejack (who happened to be in town). Applejack, on the other hand, is more familiar with Pinkie Pie's "Pinkie Sense," and warns Twilight to heed the warnings Pinkie gives. And sure enough, Pinkie's ear start to flop around, signaling that someone is going to need a bath. Twilight just laughs at this, only to get splashed by a passing cart. At least Pinkie Pie's nice enough to let her use her bathtub. As Twilight bathes, Pinkie explains the various little "senses" she gets and what they mean, such as a pinchy knee = something scary's about to happen, and an achy shoulder = alligator in the bathtub! Twilight thinks that should have been a pinchy knee, but the alligator is actually Pinkie's pet, Gummy, who has no teeth and thus isn't that scary.
as Twilight learns the hard way. Faced with such a strange phenomenon, Twilight Sparkle turns to science to give her an answer, and hooks Pinkie Pie up to a device that measures...brain waves or something. This will give her all kinds of scientific data to study on Pinkie's twitches, but of course, once Pinkie is all hooked up, she's got nothing. Pinkie tells Twilight to just believe in the Pinkie Sense, even if she doesn't understand it, but Twilight refuses to believe in anything she cannot explain. But with the device and data looking to be a big disappointment, Twilight unhooks Pinkie Pie, declaring that she doesn't need to know, nor does she even care, how this "Pinkie Sense" works. But, naturally, once set free, Pinkie gets the "watch out for doors" combo and Twilight once again gets smashed. Forget what she said earlier, she's now got a burning desire to figure out just how that darn "Pinkie Sense" works.
Since getting data through a machine didn't work, Twilight turns to secretly observing Pinkie in her natural habitat. Down at the schoolyard, she spies Pinkie scratching her nose, then taking cover, and thinks she's disproven Pinkie's "sense," as a twitchy tail means something's about to fall, not an itchy nose. But Twilight is in for a surprise when she's suddenly beset by a swarm of bees, the true object of Pinkie's sense. One round of bandages later, and she's back on the trail, keeping an eye on Pinkie over at Sweet Apple Acres, with Spike by her side. She observes the combo for "opening doors," and Spike goes into hiding, while Twilight teases him for actually falling for this stuff. She tempts fate by hanging out by the barn's door, and when nothing happens, she feels vindicated, until she falls through the door of Applejacks new apple cellar.
Even broken bones are not enough to deter Twilight, and with a little help from Spike, she continues her observations. She dictates her findings to Spike, but at the words "twitchy tail," he heads for the hills, leaving Twilight defenseless when she's suddenly struck by not just a flowerpot, anvil, and cart of hay, but a piano as well, courtesy of a certain wall-eyed pegasus muffing up a moving job. Twilight apparently heals pretty quickly, as she's able to come out and confront Pinkie Pie when she hears the pink pony telling Applejack how she's been letting Twilight follow her all day.
something sinister slithers in the background, unseen. As the others enter the bog, Pinkie's shivers get stronger, and Spike wonders what kind of fate might befall Fluttershy (exploded, and then exploded again?). It takes some searching, but eventually they come across Fluttershy, who is perfectly fine. Twilight takes the opportunity to say "I told you so," which is exactly when the sinister slithering thing makes its appearance, wriggling up behind her. Advised by the others to turn around, Twilight Sparkle comes face to face with a hideous hydra!
The ponies beat a hasty retreat, except Pinkie, who is too paralyzed and has to be grabbed by Twilight before she can make a run for it. Fluttershy apologizes to the frogs as they hop away, and the hydra rises out of the bog to make chase. The ponies are able to evade the heads, but just barely. Spike gets stuck in the muck and is rescued by Twilight at the last moment, but luckily the hydra also gets bogged down in the same spot, giving the ponies a chance to get ahead. Pinkie, however, is still shuddering as they reach a cliff. There are enough stones to get across, but is there enough time? Fluttershy remembers "a hop, skip, and a jump" from episode seven, and Twilight sends Spike across, but Pinkie's shuddering is too bad for her to jump, and so Twilight gears up to distract the hydra while Applejack makes it across with Pinkie Pie in tow.
Asking herself what Rainbow Dash would do in this situation (for some reason), she comes up with the obvious answer: CHARGE! She manages to trick the hydra into incapacitating itself, but only for a moment, and not long enough for her to get to the stepping stones. The hydra's heads lunge at her, knocking over most of the stones. Pinkie Pie, still shuddering, calls for Twilight to jump, but Twilight just can't see how she could possibly make it (and can't muster up the concentration necessary to teleport, I guess). But with no other choice, she takes the leap, helped out a bit by the hydra knocking the ledge she's standing on out, but she misses the first remaining stone by *that* much. Falling to her doom, she lands on a rising bubble of swamp gas, which launches her back up to (relative) safety. Twilight is ready to believe in Pinkie Pie's "Pinkie Sense," after encountering a hydra and her miraculous rescue (after all, who would expect that), until Pinkie Pie starts shuddering again.
Hearing that the hydra wasn't it pushes Twilight over the edge, and she literally becomes a ponyta in her rage. The others cower, waiting to see what she'll do in this state, but just as suddenly as it flared, her anger fizzles out, and she admits that she gives up. She can't understand how or why Pinkie Pie's "Pinkie Sense" works, but it does, and that's good enough for her. As Twilight finally admits that she believes, Pinkie suddenly goes into overdrive, then suddenly stands still and tells them that that was it. After all, who would have expected Twilight Sparkle to actually give up on trying to understand something? With that solved, Pinkie Pie prances off, leaving the others dumbfounded.
Sometime later, Twilight Sparkle has Spike draft a letter to Princess Celestia, but first he has to get over the fact that she's given in to the "Pinkie Sense" and is giggling it up with Pinkie Pie, the two of them wearing umbrella hats. The lesson this time: just because you don't understand something, doesn't make it not true, and you just have to choose to believe. With the letter done, Twilight and Pinkie head off, wondering what will fall this time, but only Spike sees it: Princess Celestia, come to pick up the letter in person (for some reason).
So, this episode was pretty divisive among the fans. A lot of people liked it because of the Looney Tunes-style slapstick and the action, but a lot of people hated it because it seemed to be punishing Twilight for seeking knowledge, and pushing a pro-ignorance message at the end. According to Lauren Faust, the genesis of the episode was just the idea of Pinkie Pie having a "knee's achy, it's going to rain" thing, and Twilight not believing it, and the message was supposed to come off more as "keep an open mind" rather than "just believe it, even if it doesn't make sense."
While I'm not too crazy about this episode storywise, I have to give it props for the animation. The expressions alone are pretty awesome in this ep, and the slapstick and action come across very well. It's worth watching, but I don't think it'll make my top ten.