Twilight Sparkle is called away to tend to Luna when she falls ill. As Luna grows sicker and Twilight more determined to help the younger princess, she begins to uncover dark secrets about the elements, the alicorns, and her own past.

Twilight finds herself enthralled by Princess Luna and everything she represents. As Celestia's pride and joy, Twilight struggles to repress these feelings. But an old and well-known evil surfaces again at the worst time. Can Twilight fight off the spectres of the past?