Twilight Sparkle knew what being an alicorn meant, but it never really hit her until the death of everyone she loved, including Celestia and Luna, who gave up their long lives and their powers to Princess Twilight and Princess Cadence. With Cadence, as Princess of Love and Light, raising the Sun, and Twilight, Princess of Friendship and the Night, raising the moon, Equestria has been at peace despite Celestia and Luna's departure over a thousand years prior. The loss of her friends left Twilight in a deep pit of despair.

A strange visitor from one of the many mirror worlds tries to talk her out of the depression by letting her know of a new threat to the peaceful Equestria that she and her friends fought for, but it might not be enough, and the beings from the mirror world that prey on negativity may have already got their tentacles in the Princess. Can Cadence and another unlikely royal manage to pull Twilight out of her despair long enough for them to reform a new, more powerful version of the Elements of Harmony to defeat the Sha?