This episode puts the focus squarely on Apple Bloom and ponies around her age, pretty much doubling the population of Ponyville as we've seen it so far.
This one starts at Ponyville Elementary where Cheerilee, the teacher, calls her class to order. Today's lesson is on cutie marks, a great boon for the audience at home, but probably not as informative for the class, since most of the students already have one. She illustrates her lesson with photos of herself, first as a baby with no mark, and then as a preteen with a mark. The 80's style gets a laugh from the class, but she pushes on, telling the young ponies about how her cutie mark appeared when she decided to become a teacher. Cutie marks, it seems, are not just decorative, but represent a pony's hopes and dreams, and they appear only when a pony is certain of their life ambition. Apple Bloom takes copious notes, but not everypony is as interested, as the pony next to her tries to get her to pass a note. This was actually a ruse, as the note was blank. "Remind you of anypony?" the note giver, Diamond Tiara, snickers as the camera zooms in on Apple Bloom's cutie mark-less rear.
After school, Apple Bloom is still bummed out by her lack of a cutie mark as she walks home with her friend, the stereotypical nerd pony Twist. Diamond Tiara and her friend Silver Spoon tease the two mark-less ponies, and invite them to Diamond Tiara's Cuteceanera (a play on quinceanera, a mexican coming of age party), and call them "blank flanks" to boot. Back at Sweet Apple Acres, Apple Bloom rages at the injustices of not having a cutie mark, but Applejack just tells her to be patient. She points out that she was the last in her class to get a cutie mark, but since her mark symbolizes her dedication to running Sweet Apple Acres, it was worth it. Hearing that Granny Smith and Big Macintosh were also the last in their class to get their cutie marks doesn't make Apple Bloom feel any better, until she realizes that all their cutie marks have to do with apples, so it's likely hers will, too.
Applejack goes into town to sell the farm's wares. Hoping for some instant gratification, Apple Bloom goes whole hog selling apples, from pointing out their various uses, to hounding random ponies on the street, to planting apples on unsuspecting ponies. That last trick costs Applejack the last of her wares to appease an angry customer, and she declares Apple Bloom's selling days are over and sends her sister home. Apple Bloom pouts, and so Applejack reminds her that you can't force a cutie mark, and suggests that going to the party with another mark-less pony will make her feel better. Apple Bloom agrees and goes off to see Twist to suggest they go together. However, it turns out that Twist just got her cutie mark (her calling is making sweets), but she insists they can still go together. Diamond Tiara and Silver Spoon just happen to be walking by in time to comment on this, making Apple Bloom's spirits sink.
Sometime later, Rainbow Dash finds Apple Bloom with a dark cloud hanging over her quite literally. Being on the weather team, she makes short work of the cloud and asks Apple Bloom what's up. Apple Bloom brings her up to speed on her dilemma (mostly that she wants to go to the party, but doesn't want to be the only one without a cutie mark), and Rainbow Dash offers to get her a cutie mark. She tells Apple Bloom about how she was the first in her class to get her cutie mark, which appeared after her first race in school made her realize her "need for speed." So Apple Bloom takes her up on her offer, and Rainbow Dash goes into coach mode. Her method is to have Apple Bloom try as many things as possible, one right after another, and eventually she'll find her special talent and cutie mark. Thus Apple Bloom tries all manner of things, from juggling to karate to roller derby, with little success at any of them.
As Rainbow Dash searches for an activity they haven't tried, Apple Bloom hides just in time to avoid Diamond Tiara and Silver Spoon, unable to face them without a cutie mark. Once the two ponies are gone, she bewails her inability to find something she's good at, prompting Pinkie Pie to arrive and tell her she'd be good at eating cupcakes. Inspired, she bids Rainbow Dash adieu and follows Pinkie Pie back to Sugarcube Corner.
with a little song, they mix up batch after batch of cupcakes. But baking is not Apple Bloom's forte, though Pinkie doesn't seem to mind. Apple Bloom starts to despair of ever getting a cutie mark, until Pinkie points out something on her flank. Apple Bloom rushes to the nearest reflective surface, but can't get a good view. As she guesses what her cutie mark could be, Pinkie just blows it off. It was just flour after all. Only the arrival of Twilight Sparkle keeps Apple Bloom from despairing some more, and she demands that Twilight use her magic to procure a cutie mark. Twilight refuses at first, telling her that you can't make a mark appear like that, but Apple Bloom's desperate pleas get her to give in. And so, with a little magic, Apple Bloom's cutie mark appears! Only to fade away almost immediately. Apple Bloom asks her to try again, but each attempt disappears just as quickly.
With no cutie mark and no hope of getting one in the near future, Apple Bloom resolves to just skip the party entirely. However, as she goes to leave, she finds she's already at the party. Seems she'd forgotten Diamond Tiara decided to hold her cuteceanera at Sugarcube Corner. As she tries to leave without being seen, her hiding spots are uncovered one after another, though she manages to stay out of sight. After commandeering a table, she nearly makes it out scot-free, but runs into Applejack at the last moment. Her sister inadvertently drags her back into the party, right over to Diamond Tiara and Silver Spoon. In a moment of desperation, she creates a makeshift skirt out of a tablecloth to hide her mark-less rear, and when Diamond Tiara starts digging into her for not having a cutie mark, she claims to have one, but tells them she doesn't want to show up Diamond Tiara at her own party. So Diamond Tiara lets her off the hook this time, and Apple Bloom walks off, triumphant. But as she walks off, she trips on her "skirt," not only knocking over the record player, but exposing her blank flank for all to see.
Diamond Tiara and Silver Spoon crow at her "amazing" cutie mark, a voice from under a table comes to Apple Bloom's defense. The owner of the voice, a pegasus, comes out and with her unicorn friend, reminds everyone at the party that a mark-less pony is a pony with unlimited potential. Without her life's goal stamped out for all the world to see, she could be absolutely anything, and at the very least, she isn't marked as a stuck-up libby. Diamond Tiara demands to know why they're defending Apple Bloom at her party, and they show off their own blank flanks. It turns out all three had thought they were the only ones without marks, but now they have solidarity. Finally some adults show up to counter the bullying, with Twilight pointing out how lucky they are to still be able to discover theirselves, and Applejack adding that they have all the time in the world to do it. The other young ponies flock to the three mark-less ones, impressed by the thought of boundless potential, much to Diamond Tiara's annoyance. Apple Bloom's defenders finally introduce themselves, though we already knew that they were Scootaloo and Sweetie Belle, thanks to the end credits displaying their names week after week.
Having bonded over their lack of cutie marks and generally dislike of Diamond Tiara and Silver Spoon, Apple Bloom, Sweetie Belle, and Scootaloo decide to form their own club to help them figure out what their calling in life is, and plan to call it The Cutie Mark Crusaders. As they join the rest of the party, Twilight Sparkle writes her report to Princess Celestia on what Apple Bloom learned, which is ostensibly that the thing you think will make you an outcast may very well be the thing that gets you friends. Princess Celestia manages to divine that the "thing" is a cutie mark, so cutie mark angst may well be a common dilemma in Equestria.
Pinkie Pie offers Twilight Sparkle cupcakes
Tasting Apple Bloom's cupcakes
Don't forget the party hats
Apple Bloom's defenders
Don't eat the cupcakes