In a dimension very separate from Remnant, we see a massive castle right in the middle of a kingdom of common folk.

This is the dimension of Mewni, a medieval-like society where magic is prevalent.

And today is a special day for all the Mewnans, as the Crown Princess, Star Butterfly of the Butterfly family is going to turn 15, and receive her wand from her mother.

We see the Princess herself taming a wild unicorn!

???: YEEHAW!!

Star Butterfly (The Crown Princess of the Butterfly Family! She's a rambunctious and rebellious teenager who loves fighting monsters and having fun! She's about to inherit her mother's magic wand!)

Star began to land on the unicorn's back and ride off to the castle, with her crashing through several stands of the civilians of Mewni!

Salesman: NO!! MY CABBAGES!!!

And then the tomboyish princess burst through the castle up to her parents!

King River and Queen Moon Butterfly (Star's parents! One loves fighting monsters, and the other is uptight and puts a lot of pressure on Star to get her under control!)

https://youtu.be/fDyPwwySEZI

Soon enough, Star's belongings were packed, and put onto a carriage which is being pulled by Manticores.

Star on the other hand is hugging her parents legs, and pleading desperately.

Star: NOOO! PLEASE! I CAN BE GOOD! DON'T SEND ME TO ST. OLGA'S REFORM SCHOOL FOR WAYWARD PRINCESSES!

She began to scream as a shot of her being sent on a conveyor belt into a school is shown.

Star: AAAAAAAAAAAAAAAAAAAAAAAAAAAGH!!!

River: Sweetie, we'd not going to send you there.

Star: Oh.

Moon: Yet.

River: We're going to send you to a safer dimension for you to train. Remnant.

Star: Remnant?

River: Manfred, open up the portal.

Manfred used a pair of scissors to cut open a portal with Star looking on as she says goodbye to her home.

Star: Goodbye Mewni.

Little did she know, she was being watched.

At Beacon...

Both Ozpin and Glynda are talking with both the King and Queen.

Glynda: So, you're from another dimension?

Ozpin: We should have seen this coming to be honest.

The glasses-wearing headmaster took a sip from his coffee mug.

Ozpin: And (Y/N) didn't even bring it to us this time.

Glynda: I expected that.

Star was seen in a different outfit messing with the light switch.

Ozpin: Very well, we accept Star as a student. And we have the best student to be her guide.

Later...

You were in the office.

(Y/N): You wanted to see me?

Ozpin: Yes, (Y/N), you are good with letting people from other worlds, universes, and dimensions adapt to living in a new realm, right?

(Y/N): Look at all the students of the Officer Academy, Night Raid and the Jaegers, the Demon Slayers, the Survey Corps, and the Rainbooms and Dazzlings. I mean, if it's one girl, she's not alone. She's got a bunch of new friends who might like her!

Glynda: (smiling) That's what we expect from you.

Glynda winked and you smirked.

(Y/N): Just leave it to me!

Soon...

Star: Thanks for showing me around, Mr. (Y/N)!

(Y/N): (Y/N) is just fine, Star. You just need to be aware of some people who can be crazy jerks. Or the occasional rival. Or-

???: (W/N)! FIGHT ME!

(Y/N): NOT NOW INOSUKE!

The crazy Demon Slayer who was raised by boars tried to attack you, only for Star to-

Star: I GOT YOU!

She blasted Inosuke with her wand!

Inosuke: AGH!

He began to see everything blurry and was foaming from the mouth.

(Y/N): W-Who are you exactly?

Star:

https://youtu.be/Nezs8F4F_l0

(Y/N):

https://youtu.be/dv2u5OUVZMg