When Celestia discovers the Queen of Cliffside—a golden pony with a red-streaked yellow mane, somepony very much like herself when she was a filly—she learns harsh lessons about the errors of her ways as she works to save Equestria from a curse: herself. Sunset Shimmer, for her part, is not amused. One of the pair proves more magical. Spoiler: They don't get along.
(Minor revisions made Sept 2017.)