Princess Luna must go on a diplomatic mission to the isolated city of Marem, and is tasked with choosing a suitable traveling partner to accompany her on the way. She does not choose the talented and magically-inclined Twilight Sparkle, nor the quick and loyal Rainbow Dash, nor even the strong and talented Applejack. Instead, she surprises everypony by taking Pinkie Pie with her and leaving without a word. Not even Celestia can guess the reasoning behind her sister's choice.

For whatever reason, Princess Luna and Pinkie Pie set off to find and negotiate an agreement with the mysterious community of unicorns located beyond the borders of Equestria. What they find there will impact not only Equestria's stance on foreign policy, but the very lives of the two ponies at the center of it all.