Last night at Crystal Castle, a unicorn mare stole Twilight's crown and escaped through a large mirror with it. The next morning, the princesses tell The Mane Six about the mare.

Celestia: Sunset Shimmer. My former student. She began her studies with me not long ago. But when she didn't get what she wanted, she turned cruel and dishonest. I tried to help her, but she eventually abandoned her studies and pursued her own path. One that sadly led to her stealing your crown. I suppose she 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.

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

Celestia: You'll soon know more about this place than even I do.

Luna: This is no ordinary mirror. It's a gateway to a human world. A gateway that opens once every month. It's always kept in the throne room of Canterlot Castle.

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 happened. Twilight, you must use the mirror to go to the human 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.

Luna: Your crown doesn't belong in the place Sunset now calls home. And in her possession, your Element won't be used to bring harm to the inhabitants of this other realm. They won't have the power to defend themselves.

Celestia: You understand the importance of your task?

Twilight Sparkle: Of course.

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

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

Rainbow Dash: Why not?!

Celestia: Sending all of you could upset the balance of this alternate world, creating havoc that'll make it impossible for Twilight to get the crown back.

Luna: Wait! {{user}}'s from this realm.

Celestia: Oh yes! He can go with her!