When I first heard the description for this episode, I was like "Wait, the Cakes have kids? Since when?" Since this very episode, it turns out.
The mane six gather at the hospital, eagerly peering at the newborns in the maternity ward, trying to find the Cakes' new foal. Carrot Cake shows them his new son, Pound Cake, and daughter, Pumpkin Cake. Pinkie Pie is beside herself, having two new foals to play with when she only expected one, and she leaps into the room, ready for a birthday party. The nurse on duty, however, shushes her , and kicks her out for singing a birthday song, even a quiet one. Applejack wonders how one twin can be a pegasus and the other a unicorn when both Cakes are earth ponies, and Carrot explains that there's one of each in their family trees way back. So Rainbow Dash tells Carrot about all the fun he's in for once Pound starts flying, and Twilight and Rarity warn him about Pumpkin getting random spurts of magic. Before he can respond, Pinkie sneaks in with a cake, and is given the mother of all shushings by the nurse.
One month later, the twins are old enough to crawl around and blow the noisemakers Pinke Pie gives them for their month-iversary. She delights in playing their favorite game, "Where's Pinkie Pie," but when it's time for a diaper change, she happily hands them off to Carrot Cake, then waits impatiently as Cup Cake feeds them. With the babies ready, Pinkie Pie gets back to playtime, leaving it to the parents to keep Pound Cake from, well, pounding things and Pumpkin Cake from putting everything in her mouth.
In the midst of this, the Cakes suddenly remember the incredibly important catering order they have to deliver, and rush out to find a last-minute babysitter, ignoring Pinkie's assertions that she would love to do it. Unfortunately, Fluttershy already has plans with Angel, and even the Cakes know not to mess with that bunny. Twilight has more reports to write, Applejack's dealing with a caterpillar infestation, Rainbow Dash has tickets to the Wonderbolts airshow that afternoon, and Rarity, though flattered, flat out refuses. With their options exhausted, the Cakes finally turn to Pinkie Pie, who plays hard to get, but agrees.
Pinkie remains firm in her conviction, though she wavers a bit after seeing the long list of things she'll need to do. But she catches herself and sends the Cakes off with a smile. But the minute the door closes, Pound and Pumpkin immediately start crying, and Pinkie realizes she may be in for more than she bargained for.
Pinkie tries a round of "Where's Pinkie Pie," but the babies cry on. When funny faces also have no effect, Pinkie searches her repertoire and pulls out her stand-up act, though the punchlines are lost on the foals and they start crying again. So Pinkie pulls out her showstopper number, The Piggy Dance. Which fails miserably. But in the course of her dance, she bumps into a cupboard and knocks a bag of flour on her head. This, of all things, makes the babies laugh, to Pinkie's relief.
Pinkie consults her list of responsibilities, and sees that snacktime is next on the agenda. She sets out the prepared food, but rather than eat, Pumpkin knocks her bowl down in grabbing a nearby tablecloth and Pound accidentally flips his bowl onto his head. End result: both foals start crying again, which Pinkie stops by covering herself in flour again. Next up, bathtime. Both foals run around the bathroom and hide before Pinkie manages to get them both in the tub, where they immediately start crying again. This time Pinkie distracts them with bubbles, which lasts until Pumpkin's horn pops one, of course. Pinkie turns to bath toys to stem the tears, and when one works, she piles on so many toys that they soon have the opposite effect. Flailing and crying, Pound manages to break off the faucet, drenching Pinkie. Seeing no other options, she pulls out a bag of flour, but gives that foals one last chance to stop crying on their own, warning them of what will happen if she has to resort to this. But they just keep crying, and so Pinkie drops the flour, creating a goopy mess on herself, which fortunately works.
the dreaded scent of diaper-changing-time wafts into the room. With great trepidation, Pinkie sneaks up on the foals, but they see her at the last moment and run off. Pinkie easily finds them and removes the offending diapers, but has a harder time getting the clean ones on. As in, she somehow ends up wearing both diapers. The foals run off again, but before she can chase them, she hears someone at the door, and assumes the Cakes are back early. But no, it's Twilight Sparkle, done with her work and ready to help out. Pinkie welcomes her in with open arms at first, but when Twilight tells her she figured Pinkie would need some help, implying that the earth pony can't handle it, Pinkie gets so offended that she kicks Twilight out and slams the door in her face. As soon as she does, however, Pinkie realizes that there was an element of truth to Twilight's insinuation--she hasn't been handling things very well. But she vows to turn things around from here on in!
Checking on the babies, she finds them in their bedroom, Pound jumping on the bed and Pumpkin chewing on a rubber chicken. Pinkie puts on her serious face and puts a stop to the jumping and chewing, puts the foals in the crib, and orders them to take their nap. To her relief, this works, and so she tucks them in and gives them each a kiss, then puts away the dirty diapers and rubber chicken. She turns off the lights, and just as she's about the shut the door, notices the crib is devoid of foals. Pinkie searches the room frantically, until she hears the squeak of the rubber chicken, and finds Pumpkin in the closet where she put it. With Pound still missing, she doesn't bother to take the chicken from Pumpkin, but simply deposits her back in the crib and asks her to please stay there.
Pinkie continues to search for Pound in the darkened house, but is haunted by the sound of his giggles and his elusive shadow. Finally, she spots him using his tiny wings to walk on the ceiling. Though she tries, she can't quite reach him. But if anyone knows about being in places you shouldn't physically be able to be, it's Pinkie Pie, and she climbs up to him using suction cup shoes. She manages to get him back down, but loses hold of him when she sees Pumpkin using her magic to grab her toys and put them in her mouth. Pinkie grabs them all and locks them up in a toybox, then goes after Pound. She manages to grab him in flight, but to her surprise, he's strong enough to drag her along. To her credit, Pinkie refuses to let him go, even as he drags her down the stairs, through plates of food, and back up the stairs. It's only when her tail gets caught in on a doorknob that she loses her grip.
levitate herself, to Pinkie's great dismay. The twins fly and float around until Pinkie traps them with a playpen, which she duct-tapes to the ground, claiming this to irrefutably prove her responsibility. Pumpkin, however, uses her magic to pass through the wall of the playpen and Pound simply flies up until he breaks through the tape. Seeing all her efforts going up in smoke, the stress of watching the twins becomes too much, and Pinkie Pie bursts into tears. The twins see this and do the one thing they know will bring a smile: cover themselves with flour. Pinkie has to admit, it is pretty funny.
Having reached an understanding with Pinkie (and worn themselves out) the twins take a nap for real, and Pinkie Pie drafts a letter to Princess Celestia while she cleans up the messes made by her babysitting attempt. Her lesson was twofold: babysitting is more than just playing, and wanting responsibility is not the same as being responsible. Night falls and the Cakes return, bracing themselves for the worst, but find a sparkling clean house and still sleeping twins instead. Amazed that Pinkie actually managed to be a competent babysitter, they ask her to be their go-to sitter. Pinkie balks, but relents when she hears the twins say her name.
One thing I was wondering after this episode, is can all baby pegasi fly like that, or is Pound just special? It might be that baby pegasi can fly as babies but as their bodies grow, they lose the ability until their wings catch up, but until the show answers that, we won't know for sure. Of course, the most likely answer is that the twins were tapping into Pinkie's "Rule of Funny" powers, inadvertantly turning them against her.