Princess Celestia wishes to show Sunburst something. Not a new library, but a new study filled with the works of Starswirl the Bearded. While the two are discussing the legacy of Starswirl the Bearded, Sunburst comes across a book called "The Great Starswirl the Bearded, The Two Sisters, And the Magical Vortex, a book Celestia is quick to point out is about her. Later that night, Sunburst is reading the book, citing that it doesn't real like a journal, but a legend. The book goes on to praise how great Starswirl is, and how he'll be known long after his death, a fact that is quite true.
The book opens with an account of Starswirl teaching a young Celestia and Luna in the arts of magic. Starswirl scolds and punishes Celestia for refusing to pop the bubble she had encased Luna in, which Celestia eventually does, but only to get Starswirl off of her back. Starswirl and Luna discuss the attitude of Celestia, and Luna's actual progression in the arts of magic; something Starswirl admits is very slow. Although Luna claims she's doing her best, Starswirl solemnly says 'her best' wont be much use.
Later that night, Luna sneaks out of Canterlot Castle in order to practice her magic spells so she can catch up with Celestia. She still fails to perform basic spells ad ends up burning a plant to a crisp instead of properly bubbling it. Celestia overheard her sisters escape and followed her behind her back. After she saw Luna's display of magic, she was quick to mock her sister, calling her pathetic and other such insults. In order to prove herself to her sister, Luna attempts to open a portal spell. Celestia claims Luna can't cast such a spell, but to her amazement, Luna manages to pull it off.
The portal opens up a gate between two worlds; two universes. The current reality, and a world of darkness, a world where some great evil consumed all light and good and left it as a shell of evil and darkness. To her horror, the beings on the other side of the portal begin to speak to Luna, they tell her to join them, to destroy her sister, to embrace her darkness. The portal begins to suck in Luna, Celestia desperately tries to save her sister, but isn't powerful enough, and Luna is gone.
A teary eyed Celestia runs to get help from Starswirl, although he doesn't understand Celestia at first, he realizes the severity of the situation. After Celestia describes the world she saw, Starswirl admits that their exist an infinite number of alternate realities, some realities have entities so evil and powerful that they can destroy all light in entire worlds, leaving them as dimensions of pure evil.
Celestia and Starswirl enter the Dark World to be the 'champions' of Luna and to save her from the darkness. Starswirl tracks Luna's magic and points into the genera direction she's being held. As soon as he does this, Celestia runs towards her sisters aid. She sees her sister being bound and trapped by odd, goblin like creatures. Celestia fends off the creatures with powerful magical blast and frees Luna, whose surprised Celestia loved her enough to save her. Now together, the two get confronted by a much larger goblin, one whose larger than trees. As the two cower in fear, Starswirl comes to their aid. They run away, Luna is dearly worried about the safety of Starswirl, but Celestia assures us if anyone can hold his own, it be him. They escape the dark world and embrace. After seeing the loving bond between both sisters, Starswirl proudly claims nothing can tear them apart ever again.