There stood the main characters, somewhere or another, discussing something nonspecific when Pinkie Pie remembers she's hungry. To save you the details, she came to the conclusion that she should eat Fluttershy (who was closest at the time).
Pinkie opens her mouth and snakes her cartoonishly long tounge around Fluttershy then lifted her into the air. Fluttershy shrieks, but since that was normal, it took a bit of time for the rest of the gang to realize what was happening.
Pinkie retracts her tounge, pulling Fluttershy with it. She got stuck half way through with her legs sticking out of Pinkie's mouth. Now it takes even longer for everypony else to realize what was happening; how many times do you see one of your friends disappear down another's throat. This gave Pinkie enough time to gulp down Fluttershy's legs, sending the entirely of Fluttershy to her stomach.
The girls shriek, well, all the girls besides Pinkie...and Rainbowdash! With a quick response, she snaps into action, charging Pinkie. Her plan: reach down her throat and pull Fluttershy out of there, how hard could it be?
As Pinkie was gasping for breath after swallowing Fluttershy, Rainbowdash successfully jams herself down Pinkie's throat. Her plan works too well and ends up in the same position Fluttershy was in a few moments earlier. Pinkie shrugs and gulps down this set of legs as well: the more the merrier!
Rarity faints for what appears to be the second time (we can't be sure, we only saw it once so far). Twilight starts shouting before anypony else does anything.
"Pinkie!" She started "ponies" she continued "are not" she conjectured "food!"
That's where Pinkie lost her and zoned out. Twilight kept talking, sure, but nopony was really listening. Instead, Pinkie couldn't help but notice how beautiful Rarity looked collapsed in the glistening sun like that. She was like some sort of marble statue come to life; flawless and perfect. Pinkie decided to eat her.
Pinkie's tounge snaked out again and wrapped itself around Rarity who (for the sake of time we'll say) didn't taste like much, so Pinkie didn't find the need to savor anything. In one mighty gulp, Rarity's body slipped into Pinie's mouth and down her throat to join the others. Twilight finally turns around.
"Pinkie?! What the [Buy some apples!]! I turn my back for a second and you're doing this [Buy some apples!] again?!"
Pinkie has a disturbingly menacing look on her face that Applejack notices. Applejack then came to the conclusion that the only way to keep her contained was to eat her. So she jumped onto the Pinkie mass and stretched her lips around her rediculous girth (of course she couldn't think anything but "Oh Celestia, I'm eating RD Rarity and the other one!" when passing over her ball of a belly).
After somehow managing to gulp down the rest of Pinkie, she turns (?) to Twilight.
"Twilight, you have to get the others out of her, you're their only chance!"
"Ok, here I go!"
(Well, the dialog was more meaningful than that, but that's not important)
Twilight preps the teleportation spell. She uses the teleportation spell. Nothing really went wrong there...except when she did it, Applejack (and everypony inside her) teleported into Twilight's stomach instead of out of it. (silly Twilight, you forgot to pick a target and location!)
Twilight is shocked that she is now the size of six ponies and blacks out.
[Rewind back to Rainbowdash landing in Pinkie's stomach]
Fluttershy was terrified, but relieved to see a friendly face. Rainbowdash might have failed to rescue her, but she cared so much about her that she dived down Pinkie's throat to do it! Now that's love or something!
...Fluttershy decides it would be a good idea to eat Rainbowdash.
"What? I would never do such a thing!"
FLUTTERSHY DECIDES TO EAT RAINBOWDASH.
Fluttershy eats Rainbowdash. Rainbowdash is shocked and disoriented, being swallowed now twice in a row.
Fluttershy realizes that thr taste of pony fur is amazing and decides to become a canibal for a living if she ever gets out of Pinkie.
Rarity comes down Pinkie's throat. Her body looks so sexy when it's covered in saliva. Fluttershy gulps down Rarity. Rarity doesn't taste like much. Fluttershy reconsiders her life choices.
Inside of Fluttershy, Rarity is just waking up and sees Rainbowdash. Rainbowdash feels double duped and wants to get back at somepony. She sees Rarity. She begins eating her in fear that Rarity might eat her. Rarity passes out again. Rarity's legs slips into Rainbowdash's mouth.
[The cycle is complete]
In the next episode of this bullshit...
News caster Guy Bobson in the middle of town reporting on the screaming riots in Ponyville.
"...and it seems like tragety has really struck the land. Let's see what the ponies on the street have to say about this."
Daisy: "It came out of nowhere like giant ghost!"
Rose: "It's terrible! She just keeps getting bigger and bigger!"
Lilly: "The horror! The horror!"
Guy: "It seems that if we don't act soon, we shall all be subjected to this misterious terror that-"
Bobson is lifted into the air by a giant...tounge? The camera looks up. Pinkie is Cthulhu.
[credits roll]