Long ago, the demon Nightmare Moon possessed Princess Luna. The resulting battle ended with Nightmare Moon being banished to a dream realm, but Luna was turned to stone in the process and Celestia lost her divine shell.
A thousand years have passed when the seal containing Nightmare Moon breaks.
Six mares are thrown together in a quest for both the Elements of Harmony and Princess Luna, who alone stands a chance against defeating the demon. Nightmare Moon isn't the only dark force at work and the six mares must venture far from home if they wish to win.