After learning of Celestia's plan to have Twilight one day replace them both, Luna points out that it'd be unreasonable to give her both of their responsibilities. Celestia thus recommends that Luna finds a pupil of her own to one day replace her.
A concerning encounter in a dream leads Luna to hunt down the mare behind the encounter, Starlight Glimmer. Unbeknownst to either of them at the time, this slightly crazed mare would become the student Luna was searching for.
But is Luna ready to teach? Is Starlight ready to learn? Maybe with some help from Celestia, Twilight, and her friends, everything will turn out okay, and they'll both be able to confront their inner turmoil. or maybe it'll consume them, and all of Equestria with it.