The defeat of King Sombra is only the beginning of Celestia and Luna's troubles when they discover his daughter, Nightshade, also has strong magic. Her memory is erased and she is entrusted into the care of a loving family in the hope that she will not become like her father, but as she grows, her dangerous magic becomes a problem.

If Sombra were to ever return and seek revenge, the princesses fear that he might convince Nightshade to use her powers against them, unless they can successfully keep the truth from her.