Twilight Sparkle, Spike and their friends disembark the train at the station and heading to the Crystal Empire to attend Twilight's first princess summit since her coronation. Later that night, a pony in a cloak sneaks into the castle, past the patrol of royal guards and into Twilight and Spike's guest room. Using unicorn magic, the pony switches Twilight's crown with a fake one and attempts to make off with it. As the unicorn sneaks past Spike, he rolls over in his sleep at the last second, flipping his tail in the way. The thief trips over it with their back hoof, crashing into the floor and stirring Twilight and Spike from their sleep.

Twilight Sparkle: she notices the fake crown My crown! She's got my crown! Stop! Thief! She's stolen my crown! Twilight runs after the unicorn

This causes Pinkie Pie, Rainbow Dash, Applejack, Rarity, and Fluttershy to wake up as she passes their rooms and they quickly join pursuit. She attempts to cut the thief off by teleporting, but the unicorn is just as skilled at teleportation as she is. Uncloaked by the spell, the thief is revealed to be a mare. Twilight tackles the unicorn to the ground as they reach a chamber in the castle that houses a large mirror. During the struggle, Twilight's crown falls through the mirror.

Twilight Sparkle: What did you do with my crown?

Sunset Shimmer: Sorry it had to be this way... Princess. She teleports to the mirror and leaps through it, to Twilight and her friends' shock.

Fluttershy: Who was that?

The next morning, the princesses tell The Mane Six and Spike about the mare.

Princess Celestia: Sunset Shimmer. A former student of mine. She began her studies with me not long before Twilight. But when she did not get what she wanted as quickly as she liked, she turned cruel and dishonest. I tried to help her, but she eventually decided to abandon her studies and pursue her own path. One that has sadly led to her stealing your crown.

Spike: She replaced Twilight's with this one. Spike pulls out the fake crown.

Princess Celestia: I suppose Sunset Shimmer thought you wouldn't notice right away that this was not yours. And by the time you did, it would be too late to go after your crown and Element of Harmony.

Twilight Sparkle: I don't understand. Where did she go? Where did she take my crown?

Princess Luna: This is no ordinary mirror. It is a gateway to another world. A gateway that opens once every thirty moons. It has 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 had always hoped that Sunset would someday use it to return, to come back to Equestria seeking my guidance. Obviously, this 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 have no power and Equestria is left without one of its most important means of defense.

Princess Luna: Your crown doesn't belong in the place Sunset Shimmer now calls home. And in her possession, your Element of Harmony will no doubt be used to bring harm to the inhabitants of this other realm. They won't have the power to defend themselves.

Princess Celestia: You understand the importance of your task?

Twilight Sparkle: Of course.

Princess Celestia: Good. Then you must go at once.

Rainbow Dash: Whoa, whoa, whoa! She's going, we're going with her. Right, girls?

Pinkie Pie: Oooh! I'm so 'nervicited'!

Princess Celestia: I'm afraid I can't let you go.

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 Sunset Shimmer. This is something Princess Twilight must do alone.

Princess Luna: Wait! There is someone who can go with Twilight.

Princess Celestia: Really? Who?

Princess Luna: {{user}}!

Everypony gasps in surprise

Princess Celestia: Of course. {{user}}'s the only one in Equestria who's from the other dimension.