On the night before the biggest battle the ponies have ever had to face, Rainbow Dash is looking at the stars, afraid of what is going to happen. Pinkie Pie comes along and they both talk a little, until Rainbow Dash finally confesses she wishes not to die in the upcoming battle.

Pinkie Pie decides to comfort her and promises her she will make sure that it never happens. She makes it a pinkie promise.
Her last one.

And then, Rainbow Dash wakes up in a very weird world, where Pinkie Pie is awaiting her.