Note: This is a non-canon April Fools' Day special.
Some time before the start of the story and battle on Solgell Island in chapter 1, the Nebulans transport Gigan and Megalon via space pods to the next planet to conquer on their agenda. Megalon is conscious in his metal coffin and bored out of his mind. Able to sense everyone around him but having no one to talk to or play with, he sadly wishes for a friend to have some fun with. He notices the fourth wall and reaches for it.
Meanwhile in Sugarcube Corner, Equestria; Pinkie Pie is stuck in the shop alone during a slow day. She can't leave the store unattended, but the Cakes are all out on errands, taking the babies with them, and there aren't any customers to entertain. Having killed as much time as possible baking goods until she was out of supplies, Pinkie takes her chef's hat off and lays down on the counter wishing there was someone around. She notices the fourth wall and reaches for it out of curiosity.
In the human realm from Equestria Girls, Sonata Dusk is slumping back in the back seat of the Siren's van. Aria and Adagio are being grouchy towards each other again, and everyone is too tense for Sonata to try and lift the mood. Sonata shrinks back sadly, wishing she had some company who wasn't so high strung. She notices the fourth wall and something odd going on with it, and reaches out.
Sonata's hand, Megalon's drill, and Pinkie's hoof all touch the fourth wall at the same time and break it like they never had before. The trio all wake up in a white void, Sonata still in her human form and Megalon shrunk to 3 meters tall. At first not noticing each other, when they do spot one another they are coincidentally mimicking each other's movements so much that they think they are looking at a weird mirror. Upon noticing that it was no reflection, the trio get startled and freak out. Sonata and Pinkie run in opposite directions as Megalon jumps up and burrows a hidey-hole in the floor. Upon noticing they weren't being chased, Sonata and Pinkie stop and turn around. Cautiously walking back together, they confirm they won't hurt each other; they ask the same question to the hole Megalon has dug. Knowing they meant him no harm, Megalon pokes his head out and confirms he won't hurt them if they won't hurt him. Megalon notices the two's bright and colorful design, thinking they look like something from a cheery Hasbro cartoon. Thinking his wish might have been answered, Megalon boops Sonata on the nose and says, "Tag! You're it!" before running off. Figuring out her wish had come true as well, Sonata smiles and tags Pinkie Pie before rushing off too. Pinkie cracks a grin and after a Scooby-Doo styled rev up, runs after the three to start the game.
For the next half hour the fast friends play everything from tag, to blind hide-and-go-seek; and keep-away with one of Megalon's inert grenades; until Discord poofs into existence near them. He catches and eats the grenade, then remarks that the trio messed things up. Megalon asks who he is, so he introduces himself as Discord Q. Draconequus, the Spirit of Chaos and moderator of the fourth wall. He explains that the three breaking the fourth wall simultaneously was what caused them to be pulled into this realm, and asks if they did it because of some dire crisis. They explain they just did it because they were bored. He can sympathize, given he had been trapped as a statue twice, then says in about 87 minutes 51.99008 seconds, the realms will reset and the three will be returned to where they started with no memory of what had happened. The trio become really sad that they will forget their new friends so easily. Discord takes pity on them and says that while the reset cannot be stopped, if they can arrange a "convergence", their realms will receive a crossover in the future and they will eventually meet again. Discord summons an elevator and takes them to the Hall of Media, an endless hallway with doors leading to every book, TV show, movie, game, etc. If they can find the doors leading to Equestria and Terra and find a common element between them, the crossover will happen. Bat-Mite and Deadpool once used this method to arrange a DC/Marvel crossover. Megalon comments that there are millions of doors and he can barely tell them apart even with the logos on them, but Discord says that's not his problem and disappears, reminding them the clock is ticking, though he gives a clue that they have to think of the creators and influences.
Pinkie figures it out: every piece of media is influenced by another, and just as each realm leads here, they are connected to each other. They just need to go to one world, then follow the degrees of separation until they find their own. The trio jumps through a door marked M/DC.
They wind up in what seems to be New York City, all in human form. A nearby newspaper reads "New World Beyond! Avengers vs. JLA!" and the three are wearing iconic superhero costumes, Sonata got Superman, Megalon got Spider-Man, and Pinkie Pie got Deadpool. They arrived in the DC/Marvel crossover Discord mentioned. Sonata realizes that since DC and Marvel are both American stories, they have a vague connection to Hasbro. After a bit of banter about how impractical spandex and caped costumes are for crime fighting, they set about trying to find the portal. As Megalon gets freaked out by Pinkie's Expressive Mask, Sonata gets a eureka moment and grabs the others before flying up above the skyline. Knowing where it is on logic of, "It's a comic book, everything happens in New York.", they spot the SHIELD helicarrier, startling the personnel, and dive through a portal on the deck.
They arrive in the realm of G.I. Joe: A Real American Hero. Pinkie is dressed as Snake Eyes, Sonata as Ripcord, and Megalon as Duke. Sonata explains that back in the 80's, they planned to set G.I. Joe in the Marvel Universe, and though it didn't go through, G.I. Joe is still heavily influenced by SHIELD and vice versa. They then realize they are inside a Cobra base and are quickly surrounded by gun toting mooks. Pinkie tells the others to trust her and not move. The mooks open fire, but miss every shot and end up taking each other out. Pinkie explains that mooks in these kind of shows have the worst aim imaginable and never hit named characters. Cobra sub-leader Destro, sitting in a tank, doesn't notice what had happened and starts monologuing to himself. The trio sneaks up on him and knocks him out.
The trio busts out of the building in the tank. Megalon is worried because Pinkie is driving. Even though Pinkie has never operated a vehicle before, she is confident in herself. She pulls a crude crayon drawn map out of thin air and explains that Hasbro shows reference each other all the time, and she knows where they should go to get to their next world. A few minutes later, Megan Williams, the human heroine of My Little Pony Generation 1, crosses the Rainbow Bridge on Firefly's back to return to her house. The two are shocked when a tank drives past the house and crosses the bridge.
On the other side, the tank transforms into a carriage with a cannon on top. Pinkie turns back into a pony, though in the G1 art style. Sonata and Megalon remain human, but their bodies become about 11-13 years old. Sonata is wearing a pair of pink overalls with a white undershirt, while Megalon is wearing a pair of jeans, a blue T-shirt, and a baseball cap. Sonata gets annoyed and changes her hair back from pigtails to a ponytail, and complains about Pinkie getting to go back to her true form, while Sonata and Megalon got turned into Molly and Danny Williams. Pinkie points out G1 didn't have Sirens; the closest it has are Seaponies. Sonata definitely doesn't what to become a Seapony; unlike Sirens, they are completely helpless on land. Pinkie asks what Megalon actually is, and he automatically answers in a robotic voice, "Nebulan-Hunter-Line-Prototype-Cybernetic-Beta-Series-2-Arthropod-Based-Lifeform-Megalon-!". Confused at the long description, Pinkie just opts agree, "Megalon is a megalon.", and leave it at that.
Moving on, Megalon points out they need to find My Little Pony Generation 4. Pinkie brings up that nearly every Pony Generation has a version of Spike, G1 included, so if they can find him, they can find the portal. Suddenly, Pinkie panics when she realizes they landed in My Little Pony: The Movie (1986) and they have to run, now. They ask what the problem is, and she points at the truly massive Blob Monster known as the Smooze engulfing the valley. The three run for their lives, asking how this can be considered a kid's movie. They run into Spike, practicing piano. When G1 Spike notices the Smooze approaching, he grabs Lickety Split and runs. However, the three see a portal inside the piano and jump in just before the Smooze overtakes it.
They emerge from a grand piano in a more urbanized environment. Sonata has become a Unicorn and Megalon has become a Breezie. The surroundings look very cutesy and there are mares walking around everywhere, but no stallions. A mare who looks very similar to Pinkie Pie shows up and greets them. They are in My Little Pony (G3) and she is G3's version of Pinkie Pie, who gleefully gets into a spree of gag humor with her G4 counterpart after a brief stare down. As she also has the ability to break the fourth wall via her "squink" ability, she already knows what is going on and offers to help them by giving in a ride in her hot air balloon to where they need to go. G3 Pinkie gives a brief tour of Ponyville and introduces some of the ponies on the ground and flying alongside in their own balloons. Sonata is at first put off by the saccharine atmosphere, but Pinkie Pie and Megalon get into the spirit of things. Megalon notes the surroundings are different from Generation 4, but also notes that this place has its own charm. G1, G2, and G3 may not have been perfect, but without them, G4 would not exist. They are all connected and the futures owes something to the past. Sonata is won over and relaxes. G3 Pinkie Pie sails them over to Unicornia's Crystal Rainbow Castle, which G4 Pinkie Pie states reminds her of the Crystal Empire and was likely inspired by it. Upon hearing that there is a "Princess (G3) Rarity" however, G4 Pinkie bursts out laughing at the thought of G4 Rarity being put up as royalty. G3 Pinkie Pie guides them to the portal and bids them farewell, the trio hopping through.
They reemerge in the Crystal Empire, showing they've made it to My Little Pony: Friendship Is Magic. Sonata is back in her Siren body, a bit bummed she'll lose it again soon but thinking it wasn't worth it too much without Aria and Adagio sharing the gift, Pinkie Pie is still laughing at "Princess Rarity", and Megalon gets a body that is best described as an adorable looking Changeling drone with rhino beetle characteristics. Realizing they've made it and now have one world down, one to go; Sonata and Megalon celebrate as Pinkie Pie continues laughing hysterically.
