This is how Thunderwing steals Twilight's crown goes in My Little Pony: Equestria Girls
[A guard walks through a corridor. We see Twilight's room. The door opens and a giant cloaked figure comes out and walks over to the bed. The figure knocks into a desk and a lamp falls, but the figure catches it before it hits the ground. The figure looks at Twilight, then at Spike. The figure smirks, takes Twilight's crown from the bedside desk, replaces it with a fake, and puts the real crown in a bag. The figure attempts to sneak off but trips over Spike's tail]
Thunderwing: [hits the ground] Ugh!
Twilight Sparkle: [half asleep] Huh? What? [turns and sees her crown in the figure's bag] My crown! He's got my crown!
[The figure runs out of the room and into the corridor]
Twilight Sparkle: Stop! Come back! THIEF!
[She runs past her friends and the Autobots as they wake up]
Twilight Sparkle: He's stolen my crown!
[Her friends and the Autobots gasp and give chase. The figure runs down another corridor and Twilight gallops after it. She teleports in front of the figure]
Twilight Sparkle: Stop!
[The figure teleports and its cloak gets in Twilight's face. The figure reappears behind Twilight, it's Thunderwing! Thunderwing runs off and hurries down the corridor but then sees Twilight, her friends and the Autobots right behind him. Twilight starts catching up and Thunderwing speeds up until Twilight pounces on him and they tumble into a room. There, the crown falls out of Thunderwing's bag, bounces off a wall and into a mirror. Twilight, her friends, and the Autobots gasp]
Twilight Sparkle: What did you do with my crown?!
Smokescreen: Yeah, what was that for?!
Thunderwing: Sorry it had to be this way... [teleports to the mirror and smirks] Princess.
[Thunderwing smiles before jumping through the mirror as well. Twilight, her friends, and the Autobots gasp again]
Fluttershy: Who was that?
Ratchet: I have a feeling I might know.
[The next morning]
Princess Celestia: Thunderwing. Unicron's most loyal and powerful follower.
Twilight Sparkle: The one who's been trying to get me on his side ever since we first met.
Princess Celestia: He probably thought that Twilight was going to realize too late that the fake crown was not hers and by the time she did, it would be too late go after her crown and Element of Harmony.
Twilight Sparkle: But I don't understand. Where did he go? Why did he take the crown?
[The princess take Twilight, her friends and the Autobots back to the mirror]
Princess Luna: This mirror is a gateway to another world. It opens every 30 moons.
Pinkie Pie: Sparkly!
Princess Luna: It has always been kept in the throne room of Canterlot Castle. But when Princess Cadance took over the Crystal Empire, we sent it here for her to watch over.
Princess Celestia: I hoped that Thunderwing would use it to seek my guidance but that is not what has happened.
Princess Cadance: Twilight, you must use the mirror to go into this other world and retrieve your crown. Without it, the other Elements of Harmony have no power and Equestria is left without one of its most important means of defense.
Princess Luna: Your crown does not belong in the place Thunderwing now calls home. And in his possession, your Element of Harmony will no doubt be used to bring harm to the inhabitants of this other realm. They will not have the power to defend themselves.
[Twilight's bags are placed on her back]
Princess Celestia: You understand the importance of your task?
Twilight Sparkle: Of course.
Princess Celestia: Good. Then you must go at once.
[Twilight starts to move but Rainbow Dash blocks her way]
Rainbow Dash: Whoa, whoa, whoa! She's going, we're going with her.
Pinkie Pie: [squee]
Rainbow Dash: Right, girls?
Pinkie Pie: Oooh! I'm so 'nervicited'!
Applejack: You do realize that's not a real word, right?
Princess Celestia: I'm afraid I can't let you go. Optimus Prime will go with Twilight.
Rainbow Dash: What?! Why not?!
Princess Celestia: Sending all of you could upset the balance of this alternate world, creating havoc that would make it impossible for Twilight to get the crown back from Thunderwing. This is something Princess Twilight and Optimus Prime must do alone.
Princess Luna: Time is of the essence. On the third day, when the moon reaches its peak in the night sky, the gateway will close. And once it does, it will be another thirty moons before you will be able to use it to return.
[Twilight and Optimus step through the mirror. An anxious Spike shortly follows]
Main Six and Autobots: Spike! No!
[Spike jumps into the mirror]
[Twilight, Optimus and Spike tumble through the portal. On the other side, Twilight becomes a human and Spike becomes a dog while Optimus stays the way he is]
Spike: Twilight? Optimus?
Twilight Sparkle: Spike? You're not supposed to... [gasps spotting him] Spike? Are you a... dog?!
Optimus Prime: He is. And you are a human.
[Twilight looks at her hooves and finds out they've turned into hands]
Twilight Sparkle: [screams]
Optimus Prime: Twilight, calm down.
Twilight Sparkle: What... does the rest of me look like?!
Optimus Prime: You. Only not you. Your muzzle's shrunk into a nose.
Twilight Sparkle: My muzzle? [panics but Optimus covers her mouth]
Optimus Prime: Are you going to scream again?
Twilight Sparkle: [shakes her head for no]
Optimus Prime: [uncovers her mouth] Good.
Twilight Sparkle: I'm glad I'm still me. Just my whole body seems drifferent.
Optimus Prime: So what? I'm still me.
[Twilight notices Optimus is still himself]
Twilight Sparkle: How?
Optimus Prime: Well, I just portal transformation does not work on Transformers.
Twilight Sparkle: Portal transformation?
Optimus Prime: Never mind. Where are we anyway?
[Twilight, Optimus, and Spike look around them]