Spike gets another episode to himself, and we learn a little bit about dragon physiology in the process
It's reshelving day, and Twilight Sparkle is ready! With all distractions taken care of, she removes all the books from the shelves and begins resorting them with her magic. Not long into the process, the sound of Spike's self-satisfied laughter causes her to lose her concentration and all the books come crashing down. It seems he was checking on the ripeness of a fire ruby he's been saving for his birthday (gems can apparently age in this universe, I guess), and it will be ready just in time. As Spike admires his soon-to-be birthday treat, Rarity enters the library and is instantly taken by the jewel. Though she's actually there for a book on historical fiction (to get a jump on the next nostalgic fashion trend), she can't help casting an envious eye on Spike's jewel. Though she wishes Spike well, she admires it just a little too much. The little dragon's crush gets the better of him and he bestows the fire ruby to Rarity, who rewards him with a kiss on the cheek. As Rarity bounds out of the library, Twilight commends Spike on his generous deed, while Spike vows to never wash his cheek again.
Skip to one week later, and the library is all set for Spike's birthday party. The only thing that remains to be done is wash Spike's Rarity-kissed cheek, which, true to his word, hasn't been touched by soap and water for a week. Twilight closes in with a wash cloth, and Spike tries to run, but gets caught in Twilight's short-range teleportation spell. Twilight finally gets him after accidentally teleporting in Pinkie Pie, who distracted Spike with a shower of confetti. The rest of their friends weren't far behind, all bearing presents for Spike, to his confused delight. In turns out that this is his first birthday celebrated in Ponyville, and before this, he only hung out with Twilight, so he only got a present from her. Speaking of presents, Rarity shows off her latest creation, a taffeta cape, and promises to make one for each of her friends, having been inspired by Spike's gift to her.
Spike revels in his gifts and thanks his friends repeatedly. As he wishes the party could last forever, Pinkie Pie informs him that it would have to end sometime, or when would he go get his birthday surprise from the Cakes? Hearing this, Spike rushes off to Sugar Cube Corners immediately, leaving the ponies to party without him. The Cakes are ready for him with a sapphire-encrusted cupcake, and as he heads home, Spike exults in his amazing luck: presents from his friends and a dragon-specific cupcake! In the midst of his exultation, he bumps right into Miss Cheerilee, knocking over all her groceries. As he attempts to help her without dropping his cupcake, the subject of his birthday comes up, and Cheerilee, wanting to wish him a happy birthday, gives him a snazzy hat she just happened to have with her.
Musing on this turn of events, Spike begins to see a trend, and tests his hypothesis by approaching Lickety Split, a boy in this incarnation. When the mere mention of his birthday is all it takes to get the foal's ball, Spike goes on the prowl for more "birthday presents." His assumption is proven false, however, after Junebug merely wishes him "happy birthday" and trots off. Spike persists, trying to get some of her flowers, until Twilight finds him. She admonishes him, and Spike comes to his senses. He promises to return his ill-gotten gifts, but once Twilight heads home, he goes on the prowl again, his tongue becoming more forked and another set of eyelids blinking.
Once the next day dawns, Twilight is shocked to find a pile of stuff where Spike's bed should be, proof positive that he lied to her the day before. She clears away the stuff to give him a stern talking to, but her lecture slips away when she sees Spike has apparently had a growth spurt in the night. As Twilight consults her books, Spike freaks out. He examines his new body, and notices that the world looks....different. Twilight tries to get him to remember what happened the day before, but Spike is too easily distracted by things like a globe and books, which he adds to his pile.
his voice starts changing, Twilight seeks outside help. She takes him first to a pediatrician, who, not knowing anything about dragons, sends her to the vet. She isn't much help either, though Spike does respond to her commands. With the professionals stumped, there's only one place to go: Zecora's! Zecora's diagnosis: Spike is going through puberty. Which makes sense to Twilight, except for the constant hoarding. Zecora explains that dragons are prone to being greedy, and that this actually fuels their growth. Thus, the more Spike hoards, the larger he will get, and the more he will want to hoard, ad infinitum. The only cure is to keep Spike from collecting anything more, but seeing as he made off with all the stuff in Zecora's hut while they were talking, this will be easier said than done.
Twilight races back to Ponyville, only to find Spike stealing Scootaloo's scooter. Twilight distracts him with a broom before he actually takes the scooter, but he has another growth spurt in the process and can barely fit inside the library door. Twilight traps Spike in one of the library rooms, but he just makes a hoard from all the books, much to Twilight's annoyance (she did just reshelve them all a week ago). She removes the books, and with nothing to hoard, Spike makes a door and goes off in search of more things.
With Spike running wild, Twilight turns to Applejack for help, but the farmpony is simply too amused by the thought of a baby dragon on the rampage, unaware as she is of Spike's growth spurts. But when Spike runs by with all the apples and leaves from her trees, it's time to get serious. Applejack and Twilight chase after Spike, but in trying to get him, accidentally tie themselves to a tree, which Rainbow Dash, passing by, finds exceedingly funny. Her laughter is cut short by a scream from Fluttershy, and the three ponies race to Fluttershy's cottage. Naturally, the sight of a (nearly) full-grown dragon sent her hiding in the trees, though hearing it was Spike makes it a little better, though it doesn't explain to her why Spike would steal her chicken coop. There's no time to explain, as the air is pierced by a scream from Pinkie Pie.
The others head for Sugar Cube Corners, where they find Pinkie Pie pelting Spike with anything she can get her hands on, mostly cakes. Despite her efforts, Spike steals the rest of the cakes (and that's terrible) and has yet another growth spurt, this time growing larger than the building. With his newfound size, Spike makes for Rarity's boutique and takes what he could never have before: Rarity herself.
much rampaging from Spike. Rainbow Dash and Fluttershy fly in to Rarity's rescue, though their good cop/bad cop routine just gets them flung into the river, tearing Rarity's cape along the way. Spike continues to rampage, and the Wonderbolts are called in (why them and not the royal guards or something? I always thought they were just a performing team, not an actual defense force...). They divebomb him a few times, shaving off Spike's spikes (somehow?), and Spike seeks solace on a nearby mountain. As he climbs, the Wonderbolts continue to attack him, so Spike empties his hoard from the water tower's tank to a cave in the mountain side and then uses the tank to trap the pegasi.
Spike roars with pride, but Rarity is having none of it, and (unaware that the dragon she's addressing is Spike) starts to call him out on all the terrible things he's done. She illustrates the absolute worst thing he did by tearing off her ruined cape and forcing him to look at the damage, which reveals the collar she made from the fire ruby. Spike recognizes the gem, but Rarity mistakes this recognition as mere desire, and insists that he will not have it, since it was a gift from the most generous dragon she knows and thus too precious to give to anyone, especially an exceedingly greedy brute of a dragon like him. Either Rarity's words or simply the sight of the jewel reminds Spike of how he gave it to her, and the memory of the kiss brings him back to his normal size. Unfortunately, this leaves both him and Rarity hanging in the air for a moment, with no time to explain what just happened as gravity kicks in and they plummet to their deaths. Seeing as they might not make it, Spike starts to confess his crush, but Rarity stops him.
All is well, however, as Rainbow Dash and Fluttershy take up the torn half of Rarity's cloak and make a break for their falling friends, catching them well before the nick of time. They gently let the two of them down, and everyone is pleased with a job well done. But Spike, surveying the damage he did, can't help but be bummed out. Before he can get too depressed, Rarity tells him she's proud of him, though he doesn't understand why. So Rarity explains that in the end, he was the one who stopped himself from rampaging, making him the hero of this tale, and earning him another kiss on the cheek.
As Spike writes of how it feels better to give a special present to a special pony than to get lots of generic stuff for yourself, Rarity shows off the fruits of her labor and presents all the promised capes. Spike is not one of the recipients, but as far as he's concerned, he already got something even better.
I'm probably overthinking this.