Taking a walk through Ponyville, Twilight discovers her friends and neighbors hiding from Zecora, a mysterious zebra who lives in the Everfree Forest, visits town about once a month, and is rumored to be a witch. Twilight is skeptical, as is Applejack's little sister Apple Bloom, who decides to tail Zecora and see if the rumors are true. Applejack and the rest chase her down before Apple Bloom gets herself in trouble, and confront Zecora at the edge of the Everfree Forest, near a patch of blue-flowered vines. Zecora cryptically warns them "Those leaves of blue are not a joke!", then retreats into the forest. The other ponies are convinced Zecora is trying to put a curse on them, but Twilight is equally convinced curses are nothing but superstition.The next day, however, Twilight and her friends are afflicted by a series of bizarre ailments: Twilight Sparkle's horn becomes limp and covered in blue spots, sapping her magical powersnote maybe... she was able to use her brush just fine before noticing it, and probably didn't bother afterward; Pinkie Pie is rendered practically mute after her tongue swells hugely; Rainbow Dash's wings become turned upside-down, leaving her unable to fly straight; Rarity's mane and pelt become long and shaggy, like a sheep-dog's fur; Applejack becomes shrunk to only a few inches high; and Fluttershy is embarrassed to find herself speaking in a deep, masculine voice.Twilight tries to ignore her friends' insistence that this is part of Zecora's curse and find a rational explanation for their condition, dismissing Spike's suggestion of looking in a book on "Supernaturals". Apple Bloom, who blames herself for what happened, wanders off to find Zecora again (with Applejack hiding in her sister's tail-hairs). Once they discover the Apple clan members are gone, the other ponies rush to track her down. Apple Bloom easily ditches her not-so-big sister, and the ponies' various impairments make finding Apple Bloom in the Everfree Forest harder than usual.The gang eventually regroups at Zecora's hut, and things start looking suspicious enough that even Twilight is convinced Zecora really did put a curse on everyone, and is now plotting to make stew out of Apple Bloom. They confront Zecora, spilling her brew in the process, and demand to know what happened to Apple Bloom.Turns out she's right behind them, safe and sound. Twilight's original guess was right: there was no curse. Their condition is the result of exposure to the blue-flowered vines they were standing in earlier, which are actually "poison joke", a magical plant with a weird sense of humor. Zecora was actually working on a cure, out of a copy of the same book Twilight had earlier rejected: it seems Twilight had misread the title of "Super Naturals", a book on natural home-remedies. With a lesson in not judging a book by its cover doubly-learned, Zecora offers to mix up a new batch of the poison joke treatment for the grateful ponies... once they convince the other citizens of Ponyville that Zecora is not only harmless, but actually quite friendly, allowing her to buy the last few ingredients she needs in town. Now a welcome member of the Ponyville community, Zecora gladly allows the Ponyville residents to use her remedy for themselves.
Tropes present in this episode (YMMV tropes can be found here):
Adult Fear: At the end, when all the ponies are taking the bath remedy at Ponyville's spa, Applejack appears to be missing. They all panic, thinking she drowned. She was really in a tub to the side, re-grown, the tiny tub now stuck to her flank.
Arbitrary Skepticism: "Curses don't exist" said the magical talking unicorn to her talking horse friends and dragon assistant, in the land populated with dragons, gryphons, and manticores. (This is lampshaded during the episode.) Magic A Is Magic A is a major belief of Twilight's, which she has to re-think in this episode and another centered on Pinkie Pie's 'Pinkie Sense'.
Aside Comment: When Spike is coming up with nicknames for the cast after the "curse" takes effect, he comments to the audience that he can't really work with Twilight Sparkle's name.
Characterization Marches On: This is the first major appearance Apple Bloom has with her big sister, though the chemistry is somewhat different. Applejack is rather condescending and overprotective of Apple Bloom, who is ultimately portrayed as even more of an Only Sane Mare than Twilight, compared to her much more impressionable and hyperactive persona in later episodes, with Applejack being the voice of reason between them.
Does This Remind You of Anything?: Twilight's horn goes limp and because of that she apparently becomes incapable of performing something she takes great pride in her prowess and stamina in.
Zecora speaks mock-Swahili, wears African jewelry, has an African spiral design for a cutie mark, decorates with African masks, and is a zebra: an African species. Ponies don't allow her to enter their stores.
Double Aesop: Don't judge a book by its cover — even literally. Twilight was skeptical about the poorly informed negative assumptions about Zecora from the start almost to the end, likely along with any Genre Savvy viewers, but the book was slyly put in as something that made her (and potentially us) make the same sort of mistake all the same.
Dramatic Thunder: Subverted. It was just Spike, accidentally dropping pans in the kitchen.
Dude, Not Funny!: Two In-Universe examples. First, Twilight not taking kindly to Spike's joking nicknames for the ponies' Poison Joke-afflicted ailments. Of course, it's later deconstructed at the end when the Mane Six learn that the effects are just harmless, even if somewhat ironic, jokes played on them. Second, albeit unintentional, when Twilight realizes that the effects of the Poison Joke are just little jokes played on them, the shrunken Applejack takes great offense to the "little" part, most likely due to being overly sensitive about being small.
Exact Words: If you pay attention, Zecora's words to the Mane Six when they meet at the forest for the first time explicitly warn them of the flowers they're standing on. Nothing about curses or any sort of consequence from her. Zecora herself points this out, with complimentary Flashback.
Fantastic Racism: Zecora is feared by the ponies of Ponyville, who close their shops and get off the streets when she comes to town once a month. Very slightly softened by the fact that the primary reason for this isn't because she's a zebra, but because she lives in a creepy forest and thus is thought to be a witch (in fact, only one of the mane cast even knows what a zebra is at the start of the episode). But while this is not really intentional, the "don't judge people by their appearance" lesson is given loud and clear.
Also, subverted with Twilight, who knows what a zebra is perfectly well and doesn't judge her based on that. It's how Zecora's actions appeared that tricked her.
Humble Pie: The moral of this episode ties in with the Mane Six realizing how horribly they have misjudged Zecora; and while Twilight is the only one to outright apologize to Zecora, the other five's meek expressions following said apology make it clear that they feel the same way.
The Nicknamer: Spike gives all of the mane ponies nicknames to fit their embarrassing conditions.
Spike: This is hilarious! Look at all of you. We got "Hair"-ity, Rainbow "Crash", "Spitty" Pie, Apple-"teeny," Flutter-"guy", and... [turns to Twilight] I got nothin'. "Twilight Sparkle"... I mean, seriously. I can't even work with that.
Pinkie Pie: And that wicked Enchantress, Zecora, lives there doing her evil... stuff! She's so evil, I even wrote a song about her...
Rainbow Dash: Here we go...
Later in the episode, when Pinkie Pie has lost her ability to speak due to a curse, she asks Fluttershy, who has been cursed to have a comically deep voice, to sing it for her.
Town with a Dark Secret: It's more of an unconscious thing, but for a town that is that consumed with unjustified prejudiced fear of a zebra, it's likely not a fact they would spread around about themselves after their collective Jerkass Realization.
The Unintelligible: Pinkie Pie, who gets a swollen (and spotted) tongue after being exposed to the "poison joke" flower.