Starlight Glimmer is sent to solve a friendship problem between Princess Celestia and Princess Luna; when it's clear that the problem is that the royal sisters don't appreciate one another, Starlight impulsively switches their cutie marks!
My Little Pony: Friendship is Magic Season 7 Episodes...
"Get your muzzle out of those books and make some friends!" That's what Princess Celestia tells Twilight Sparkle. She may be the smartest unicorn in Equestria, but Twilight Sparkle gets an "incomplete" in friendship. There's more to life than learning magic, after all - so she goes to Ponyville on a mission to make friends. There she meets five special ponies who take her on exciting adventures and teach her the most powerful magic of all... the magic of friendship!
My Little Pony: Friendship is Magic - A Royal Problem Synopsis and Schedule