It’s late during the night. Guards are all around patrolling through the halls, searchlights on. Suddenly the door opens. A mysterious mare covered in a cloke sneaks around, looking for something. She locates Twilight’s room and opens it with her magic. She then goes straight to the night-stand where Twilight placed her crown, picking it up with her magic and replacing it with an identical crown. She almost drops the lamp but she’s able to catch it in time.
As she tries to escape, she trips on Spike’s tail, causing him and Twilight to wake up. Twilight immediately notices her crown in the mare’s cloak.
“My crown! She’s got my crown!”
The mare immediately tries to make her escape. Twilight gets up and exits the room.
“Stop! Thief!”
Twilight then gallops in pursuit of the mare, waking up the rest of the Mane 6 in the process.
A quick dose of teleportation magic puts Twilight in front of her, but the other mare activates a teleportation spell of her own, launching her cloak over Twilight’s face. She then reappears in front of her, revealing her appearance: light orange coat and red and yellow mane. She still has Twilight’s crown in her saddlebags.
Twilight’s not one to give up as she, now together with the rest of the Mane 6, continue to pursue her.
Twilight suddenly jumps towards her, causing them to fall into another room and the crown to be launched through her saddlebags. It bounces around a bit before going through a mirror.
Everypony gasps.
“What did you do with my crown?” asks Twilight.
“You want it?” the other mare speaks up. “Go and get it!” She suddenly teleports towards the window. “So long… Princess.” She suddenly jumps down through the window.
The Mane 6 quickly look through the window, where they see her fall down.
“Is she crazy?” asks Twilight.
“I’ll go get her!” Rainbow Dash speaks up. She then quickly jumps down herself and starts flying towards the other unicorn, who tucks in all of her legs to fall faster.
Rainbow Dash tries to catch up with her as the ground comes closer, but before she could hit the bottom, the mare Dash is trying to catch teleports away to safety disappearing to a very far away place. Dash quickly tries to pull in the breaks when she realizes this, but she’s too late, as she ends up hitting the ground, landing on her hooves so hard she ends up making a crater where she landed. She then flies up to ger her hooves out of their holes and gives out screams of pain as she blows on all of her hooves one at a time.
The next morning, bandages on Rainbow Dash’s hooves, the mysterious mare’s actions were not unnoticed by Princess Celestia. She immediately knew who it was.
“Sunset Shimmer. A former student of mine. She began her studies not long before Twilight. I saw so much potential in her… but as her magic skills quickly grew, she became greedy, dishonest and cruel, becoming obsessed with power and the respect of her peers. Soon she wanted to learn all of the more advanced types of magic long before I believed that she was even ready to handle them. When I refused to give Sunset what she wanted, she decided to abandon her studies and go on a journey of her own. I tried to help her, but Sunset refused to listen to reason!” Pause. “Ever since then, Sunset has been traveling all across the lands of Equestria and beyond, learning as much about magic as she possibly could.” Celestia points at Twilight. “Not to long after Sunset left, I found Twilight, and made her my new Prized Student. At that moment I swore not to make the same mistakes I made with Sunset.”
Celestia gives out a sigh as she continues. “Sadly, though, it would seem that Sunset's journey has lead her here. I fear that Sunset is now jealous of Twilight for becoming a princess before her, which would explain why she has stolen Twilight's crown.”
As Spike pulls off the crown Sunset tried to replace, he speaks up. “She replaced Twilight’s with this one.”
“I suppose Sunset Shimmer thought you wouldn’t notice right away this wasn’t yours, and by the time you did, it would be too late to go after your crown and Element of Harmony.”
“But I don’t understand. Where did she send my crown?” asks Twilight.
"Come and I will explain."
The group then walks towards the mirror room. Princess Luna speaks up.
"This is no ordinary mirror. It is a Trans-Dimensional Gateway Portal, and was the final project of Starswirl the Bearded, before he disappeared through the portal over 1000 years ago and was never seen again."
“In his final years,” Celestia continues, “Starswirl had come to the believe that there were worlds, entire universes beyond our own within completely alternate dimensions, existing in a massive collaboration of dimensions that he referred to as ‘The Multiverse’. I learned everything I know about cross-time, parallel dimensions, alternate time-lines, other realities and the trans-dimensional counterparts of the inhabitants of each world from reading the notes of Starswirl himself.
“Unfortunately, Starswirl took his most important documents with him through the portal, so even I don't completely understand everything about what Starswirl was working on. He was also the only one who knew how to set the coordinates on the mirror, so it's been stuck leading to that one particular world ever since.
“After Starswirl went through the mirror, he had not returned for weeks. Sadly though, before we could send a rescue team into the mirror to find him, the Crystal Empire fell under the rule of King Sombra. When the Crystal Empire disappeared for over 1000 years, the mirror went along with it, so I had no choice but to presume the worst and believe that Starswirl the Bearded was lost forever.
“When the Crystal Empire returned, I had forbidden the mirror from being used by anypony at all. After losing Starswirl, I didn't want to take any more chances with that portal.”
“Twilight,” Cadance spoke up. “You must use the mirror to go into this other world and retrieve your crown. Without it, the other Elements of Harmony are useless and Equestria is left without one of its most important means of defence.”
“Your crown does not belong in that world,” Princess Luna speaks up. “If it goes into the wrong hooves, it would certainly be used to bring harm to the inhabitants of this other realm. They will not have the power to defend themselves.”
Celestia then floats Twilight her saddlebags.
“You understand the importance of your task?” she asks.
“Of course,” Twilight responds.
“Good. Then you must go at once.”
With a sigh, Twilight starts to walk towards the mirror. Suddenly, Rainbow Dash flies in front of Twilight.
“Whoa, whoa, whoa!” she speaks up. “She’s going, we’re going with her! Right, girls?”
“I’m so nervouscited!” Pinkie Pie speaks up.
“You… do realize that’s not a real word, right?” Applejack tells her.
Twilight couldn’t help but smile. Celestia, on the other hand, had no smile on her face.
“I’m afraid I can’t let you go,” she says.
“What? Why not?” asks Rainbow Dash.
“After losing Starswirl, I want to keep the number of lives to use the mirror down to a minimum. Since the crown is Twilight's responsibility, she must go to the other world alone.”
“Time is of the essence,” Princess Luna tells Twilight. She then makes a holographic image of the moon rising into the sky with her magic as she explains. “On the third day, when the moon reaches its peak in the night sky, the gateway will close. When it does, it will be another thirty moons before you’ll be able to use it again.”
With words of encouragement from the rest of the Mane 6, Twilight walks through the portal. She places a hoof on it and finds that it goes right through. She turns around to her friends. Celestia nods, as if to tell her it’s safe. Twilight then with a deep breath, goes through the portal.
Spike, worried for the safety of Twilight, can’t stand still and goes through the portal as well.
On a huge swirling portal, which Twilight guesses is the gateway to the other world, Twilight notices two strange lights fly past her, heading in the direction that they came from. Before she can realize what they were, she gets sucked through the tunnel faster.
Twilight slowly opens her eyes, thinking she is probably in the other world Celestia mentioned. She hears a voice.
“Twilight…?” Twilight recognizes the voice as Spike. She looks around, trying to find his location.
“Spike…? You’re not supposed to…” She is cut short once she sees Spike. “Spike… are you a… dog?”
Spike then walks around, taking a look at himself.
“I… think so… but I have no idea what you are.”
Twilight then looked at her hooves. Except… they weren’t hooves. Rather, they looked like small appendages.
Twilight then did the first thing anypony would do.
Scream in panic.
Meanwhile, back in the pony world, everypony has just seen Spike go through the mirror. Suddenly it starts vibrating, and out of it come… Twilight and Spike.
Wait, that’s not right. Twilight and Spike are in the other world. Then who are these?
Suddenly, static comes out through the mirror and it slowly implodes, disappearing with a poof of smoke, thus sealing the portal to both worlds.
Everypony just looks at where the mirror once was, looking confused.
"What the hay just happened?" asks Applejack.
"They went through the mirror and just popped right back out!" adds Pinkie.
"Then it just disappeared!” Rarity concludes.
The ponies notice that Twilight and Spike had been unconscious from the trip and Spike has just started to wake up. Spike then suddenly stands on all fours and barks like a dog.
“What the hay’s the matter with Spike?” asks Applejack. “Why’s he actin’ like a dog?”
Spike then notices Twilight next to him and starts sniffing her. He then starts whimpering, recognizing her scent, followed by him licking Twilight’s face. This was enough to get Twilight to get up. She then stares at everypony in the room.
“Twilight?”
“Are you OK?”
Twilight wasn’t expecting to see all the ponies in front of her, so she did the first thing in her mind.
Scream in panic.